#elementary-number-theory

1 messages · Page 59 of 1

sterile pumiceBOT
alpine jasper
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i stared at it for like 2, 3 mins and i got nothing

light flicker
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what happens when you get to

alpine jasper
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is there something useful when looking at element raised to the divisor of its order?

light flicker
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$a^{k \frac{p-1}{k}}$?

sterile pumiceBOT
alpine jasper
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oh

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i reached 1

light flicker
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indeed

alpine jasper
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and so everything after repeats

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..

light flicker
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yep

alpine jasper
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and since a has order p-1

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everything before k does not repeat

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hmm did i do something stupid with my examples?

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,rotate

sterile pumiceBOT
alpine jasper
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like, for 17 and 19, something repeats before kth element

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oh

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i didn't start with a^((p-1)/k)

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thank you zoph

light flicker
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any time

azure pendant
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i have a pretty baby question but i cant see how to make any progress on it. i'm supposed to find the least residue of $10^{2019}$ mod 1009. it's pretty easy to factor out a 3 and show that this is the same as $(-9)^{673}$ mod 1009, but i'm not sure where to go from here

sterile pumiceBOT
azure pendant
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the problem is written in a way that makes it seem like 1009 being prime is important, but im not sure how to use that information

fervent crest
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Fermat's little theorem

azure pendant
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oh

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we havent learned that but i can probably work through the proof in my solution

fervent crest
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So Fermat's little theorem states that if $p$ is prime and if $\gcd(a,p)=1$, then $a^{p-1}\equiv1\pmod{p}$

sterile pumiceBOT
sacred junco
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Yeah there’s no way to do this w/o fermat

fervent crest
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There is always another way

winter bear
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indeed there is

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u manually calculate (-9)^673 first

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in Z

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and then take the mod

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ez

torn escarp
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the only alternative I'd consider legitimately is writing 673 in base 2 and then work out by repeated squaring but even then 🤢

light flicker
sacred junco
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sorry

sacred junco
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if we have f(x) = 1988x^2 + bx + 8891, and g(x) = 8891x^2 + bx + 1988, why is it that any factor of g or f must be a factor of g(x) - f(x)

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i think i may be missing the obvious here

light flicker
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its not obvious

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you have to use the fact that the polynomials are kind of symmetric to one another

sacred junco
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so if the quadratic and constant coefficients were different, this would be not true?

light flicker
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it says or

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whoever

sacred junco
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why did you delete your messages @fervent crest

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?

light flicker
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yes you're right

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its only because you get the polynomials from switching the constnat and quadratic terms that this works

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if it said that any factor of g and f must be a factor of g - f

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then this is always true, regardless of g and f

sacred junco
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what if it said some (or more) factors of g must be factors of f

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but not necessarily all?

light flicker
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uh

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idk, that's a weird statement to make

sacred junco
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well what about what @fervent crest said

light flicker
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I guess its always true since 1 is a factor of g, and is always a factor of f

sacred junco
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before they deleted it

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it was something like

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f(x) = q(x) * h(x) and g(x) = p(x) * h(x)

fervent crest
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lol i deleted because it wasn't the question

sacred junco
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then g(x) - f(x) = h(x)(p(x) - q(x))

light flicker
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I mean, this is what I said

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in this case, h divides both g and f

sacred junco
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okay so the original polynomial difference was g(x) - f(x) = 6903x^2 - 6903 = 6903(x^2 - 1)

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so i suppose i'm a little bit unclear on why this means x^2 - 1 is a factor or something?

fervent crest
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Now I don't think the statement is true tbh

hasty jacinth
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If it is a factor of f or g, and a factor of g-f, then it is a factor of the other

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Implies f and g has the same roots?

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But they dont have the same roots..

sacred junco
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i am just really lost right now ngl

light flicker
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yeah, this isn't true

sacred junco
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what is not true?

light flicker
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your original statement

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"that any factor of g or f must be a factor of g(x) - f(x)"

sacred junco
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sorry

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it's missing 1 important word

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"any common factor of g and f must be a factor of g(x) - f(x)"

light flicker
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oh

winter bear
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g and f

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not or

sacred junco
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so now it's true?

light flicker
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yes

sacred junco
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okay. could you explain why?

light flicker
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this is true in general for any ring

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just write out the definition of divides

sacred junco
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g(x) - f(x) = h(x)(p(x) - q(x)), so is 6903 h(x) in this case?

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or is it (x^2 - 1)

light flicker
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h is anything that is a common factor of g and f

olive remnant
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anyone know how to solve this its driving me crazy

sacred junco
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alright thanks

olive remnant
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welp

light flicker
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what do you know about these types of problems

olive remnant
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that's not what i thought that was going to happen

light flicker
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{ }

olive remnant
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i know euler's function

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3^(71)^82

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and i guess i know like how to find inverses with like

light flicker
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euler's function?

olive remnant
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like phi

hasty jacinth
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Phi

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The totient one

olive remnant
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where it gives like the # of coprime numbers less than it

light flicker
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And why do you think that helps you here?

olive remnant
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maybe like

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reduce down the 82 first

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by doing (3^71)^82 (mod phi(23))

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where im modding the 82

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idk if that's a word

light flicker
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And why would this help?

olive remnant
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bc maybe then i would get a smaller number to work with so i could just brute force it lol?

hasty jacinth
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23 is already prime

light flicker
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Does this preserve your original number though?

olive remnant
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i think?

light flicker
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how do you know that $3^{72^{82}} \pmod{ 23} = 3^{72^{82}} \pmod {\phi(23)}$?

sterile pumiceBOT
olive remnant
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well the exponent gets modded by phi(23) but the number sa a whole is modded by 23 still

light flicker
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by the way, $\phi(23) = 22$ since 23 is prime

sterile pumiceBOT
olive remnant
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yeah

quick furnace
light flicker
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Yeah okay, that's the right idea

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So we can reduce down the exponent, which is $72^{82} \pmod{\phi(23)}$

sterile pumiceBOT
light flicker
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This is called Euler's theorem by the way

olive remnant
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oh okay

light flicker
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Okay, now what

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(you should also try to understand why this is even allowed)

olive remnant
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tbh idk after that point :/

light flicker
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well okay, we know that $\phi(23) = 22$ so its just

olive remnant
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ive been trying just random brute force stuff for an hour lol 😅

sterile pumiceBOT
light flicker
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$72^{82} \pmod{22}$

sterile pumiceBOT
light flicker
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what's one way we can simplify this? How would you simplify $72 \pmod{22}$?

sterile pumiceBOT
olive remnant
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6 mod 22

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i think

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so 6^82

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?

hasty jacinth
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You can do better than that

light flicker
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And why can we do that?

olive remnant
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6^16 mod 22?

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uh not really sure

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i've kinda just been doin it

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lol pepeNerv1

light flicker
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Can we reduce down the exponent mod 22? Are we allowed to do that?

olive remnant
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yes bc of euler's theorem?

light flicker
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What does Euler's theorem say

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I.e., what did we do earlier?

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Did we reduce the exponent mod 23?

olive remnant
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phi (23)

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we reduced it by phi 23

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so it becomes mod 22

light flicker
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So what do we need to do here, to reduce the exponent

olive remnant
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and thats what we're reducing the exponent by?

light flicker
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But the exponent is $72^{82}$

sterile pumiceBOT
light flicker
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so we're trying to find $72^{82} \pmod{22}$

sterile pumiceBOT
olive remnant
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oh

light flicker
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and we need to reduce this somehow

olive remnant
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ohhhh

light flicker
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and it'd be nice if we could reduce the exponent of this

olive remnant
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im not entirely sure how to do that ik the bottom is 6^82

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but now i see that we can't just do the euler's stuff to the power

light flicker
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and why not?

olive remnant
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bc 22 isn't prime

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or coprime at least?

light flicker
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Does that matter?

olive remnant
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coprime to uh

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22 and 6 aren't coprime

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so we can't use euler's theorem i think

light flicker
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Yeah

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So we want to split this into things so it is coprime

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Do you know of anything that could help us with that

olive remnant
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can't we just do 3^82 * 2^82

light flicker
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but are those both coprime to 22?

olive remnant
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no i thought we'd just do one of them and worry about the other one later lol

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in that case i have no clue :/

light flicker
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Then sure, we can

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so which one can we do

olive remnant
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3

light flicker
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and what do we get when we do it

olive remnant
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3^16

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which is 5

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mod 22

light flicker
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uh, how'd you get that?

olive remnant
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uh i did (3^4)^2

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and i just did 3^4 mod 22

light flicker
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how'd you reduce it down to 3^16?

olive remnant
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and then squared it and put it through mod 22 again

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oh

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oh right

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phi(22) is 10

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3^2

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so its 9 mod 22

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mb

light flicker
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yeah

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now for the second part

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any ideas?

olive remnant
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uhh

light flicker
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if we write that $x = 2^{82} \pmod{22}$

sterile pumiceBOT
light flicker
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then we know that $x = 2^{82} + 22k$ for some integer $k$ right?

sterile pumiceBOT
olive remnant
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yeah

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so that's euclidean algorithm right

light flicker
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uh

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not really, this is just the definition of mod

olive remnant
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o

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i thoguht maybe we had to use euclidean algorithm at this point

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oops

light flicker
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well okay, is there anything you notice about x here?

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Like some condition x has to satisfy?

olive remnant
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it has to be even ig?

light flicker
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yeah exactly

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so we can write that $x = 2y$ for some integer $y$

sterile pumiceBOT
light flicker
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so that $2y = 2^{82} + 22k$

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now what

sterile pumiceBOT
olive remnant
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now y= 2^81 + 11k

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so that's uh y = 2^81 (mod 11)

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and then its like uh 2

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bc 81 mod phi(11) (which is 10)

light flicker
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yeah

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exactly

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now bring everything together

olive remnant
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wait but how i have a 9 mod 22 and a 2 mod 11

light flicker
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but $y = 2$ so what is $x$

sterile pumiceBOT
olive remnant
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oh so x = 4

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wait

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no

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im st0pid

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x = 1

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so its just 9 in the exponent?

light flicker
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$x = 2y$

sterile pumiceBOT
olive remnant
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o

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so its 4

light flicker
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(x had to be even remember)

olive remnant
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yeah smh im stupid mb lmao

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so 36 in the exponent which reduces down to 14

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bc the exponent was mod 22

light flicker
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yes

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still a bit of calculation to do but its a bit easier

olive remnant
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so now we have 3^14

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and im good from there i think

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thanks!

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sorry for being a bit slow lol

light flicker
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you're fine

olive remnant
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i haven't had the time to focus on number theory bc my discrete and multivar classes were harder than i thought they would be B(

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but thank u very much for the help

light flicker
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any time

sacred junco
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hey, I have a doubt regarding definitions: does a prime element always has to be irreducible as well?

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I know that in an integral domain that's always the case, but it is not strictly required by definition, right??

light flicker
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Yes

sacred junco
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okay, thanks!!

sacred junco
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I solved the a part by proving b and c part

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I wanted to know if there is a way to directly prove the a part

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(also because that's what the author wanted us to do)

light flicker
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how did you do part b and c?

sacred junco
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for part b, as any field with 0 characteristic has subring isomorphic to Z, and so will have a subfield isomorphic Z's field of fractions, Q

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for c I used the map :

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$\phi (x) = 1.x$

sterile pumiceBOT
sacred junco
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where x element of $Z_p$

sterile pumiceBOT
light flicker
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hm, what's the definition of prime subfield?

sacred junco
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a prime field is one which has no proper subfields

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a prime subfield is one which has no proper sub-subfields

light flicker
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ah okay

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Well I guess one problem would be that you haven't done part (a) in showing that this is the unique prime subfield

sacred junco
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ah okay, you mean I have shown only existence, not uniqueness?

light flicker
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yeah

sacred junco
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I think it can be proved that's it's unique in the sense that all other subfields will have to contain the subfield formed from the image of the mapping

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I mean all subfields will have 1, and thus will have 1, 1+1, 1+1+1...

light flicker
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yeah

sacred junco
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does this help in anyway in proving part a) ?

light flicker
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wdym?

sacred junco
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I wanted to know anyway to directly prove part a) without first proving part b) and c)

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I proved only existence for part c) but now also got to uniqueness

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does this help in part a)?

alpine jasper
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i tried it with x^4=-1(p) but i don't know how to argue that (p-1, 4) = 4, i tried to assume that it equals to 2 and get a counter example but that didn't really worked out

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@light flicker my nt papa help pls

light flicker
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okay, well if (p-1,4) = 2, then you know that p is 1 mod 4 right

alpine jasper
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yes

light flicker
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so you just have to show its not 5 mod 8

alpine jasper
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actually, if i pick p=7 then that breaks it?

light flicker
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wdym breaks it

alpine jasper
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the implication (p-1, 4) = 2 implies p = 1(4)

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logic is not my strong suit

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so you just have to show its not 5 mod 8
p != 5(8)?

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i don't understand how you arrived at that

light flicker
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the statement (p-1,4) = 2 is the same as (p-1,2) = 2, and so it just becomes the first part

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if p is 1 mod 4, then it is either 1 mod 8 or 5 mod 8

alpine jasper
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oh

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hmm

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alright let me see if i can prove that

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uhh idk man,
if i assume (p-1, 4) = 2
then that's iff p = 1(4)
and i'm suppose to show that this implies(iff?) to p = 1(8)

light flicker
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indeed

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Again, p = 1(4) iff p = 1 (8) or p = 5(8)

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can a prime p satisfy both p = 5(8) and (p-1,4) = 2?

alpine jasper
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hmm probably not, let me try to prove that

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4 does not divide p-1 hence 4 does not divide p-5, hence 8 does not divide p-5

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sounds ok..?

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wops

light flicker
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uh

alpine jasper
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i'm eating rn, sorry but maybe i should finish eating first

light flicker
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yeah, that's right

alpine jasper
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ty

alpine jasper
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how can i figure out $x^7\equiv2(49)$ has any solution without solving it?

sterile pumiceBOT
alpine jasper
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@light flicker vvSmug

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so i was trying to use this but p = 7, and 7 divides 7

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i don't know if there are any other theorems out there

light flicker
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i dont think its easy

winter bear
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i mean i'd just say

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that x^42=1 so 2^6 must =1 which is contradiction, but this doesnt generalize that nicely i think

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2 and 6 are small numbers here

alpine jasper
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hmm

light flicker
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oh no thats nice

alpine jasper
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wow

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that's a pro fucking gamer move

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wait actually i don't quite understand why x^42 = 1

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oh wait phi(49)=42

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nice

alpine jasper
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actually, how do you know x^42 = 1? i thought i need prime to apply FLT @winter bear

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wops

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eulers theorem

winter bear
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x^42=1 for all x in Z/49Z*

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yeah

alpine jasper
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am retarded

torn escarp
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looks like Hensel's lemma

alpine jasper
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yeah, my next question is apply hensel's lemma to calculate some stuff

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anyway... new question

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so i'm trying to do the same for $x^7\equiv18(49)$\
i rise both sides to 6th power and since $18^6\equiv1(49)$, i get $x^{42}\equiv1(49)$, which is true by euler's theorem, this means i can pick any $x$ that is co prime to 49 and it'll work

sterile pumiceBOT
alpine jasper
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but it doesn't

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the pattern seems to be +7, which is the power

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what is going on retard

winter bear
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this garuntees the existence of a solution

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so basically use the fact that Z/49Z* is cyclic

alpine jasper
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so i find a generator/primitive root

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?

winter bear
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i mean if you want the explicit solutions sure

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to show existence/number of sols u dont need that

alpine jasper
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i know, but know more is better than know less

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can you explain why this won't give me solution?

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did i alter the congruence when i rise both sides by 6

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i guess i surely did

winter bear
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um suppose g is a generator of the multiplicative group

alpine jasper
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uh huh

light flicker
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your implications are basically going backwards

winter bear
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we are saying that since 18^6= 1(49), that this thing has a solution right, cause 18=g^n for some n and g^(6n) = 1 which means 7|n

light flicker
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just because x^42 = (18)^6 doesn't mean you can just take 6th roots

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and say that x^7 = 18 for all x

winter bear
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yeah

alpine jasper
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okay let me digest john's message, i'm big dumb at algebra

winter bear
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thats proof of existence btw

alpine jasper
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gotcha

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hmm alright

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how does the logic work here if i'm going backwards?

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i showed that that congruence leads to euler's theorem

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uhh...? but F => T is still true

winter bear
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yeah that wont work here, which is why i wrote that thing

alpine jasper
winter bear
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if 18=g^n and since 18^6=1, then g^6n = 1 which means 7|n as order of g is 42. this means 18 is a 7th power

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the fact that these groups are cyclic are very powerful for precisely this kind of manipulaitons

alpine jasper
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i don't see how that implies solvability retardpepe

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can you explain more please

winter bear
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ok erm

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do you agree with 18 is 7th power

alpine jasper
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yes, since 18 \equiv x^7 right

winter bear
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no using the generator argument i used

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forget about the x^7 for now

alpine jasper
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yes

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i agree that 18 is the 7th power

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w/ the generator method

winter bear
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so if 18 is a 7th power

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then does x^7=18 have a solution

alpine jasper
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i suppose that allows me to take 7th root on both sides?

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oh wait yes duh

winter bear
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the root isnt going to be unique

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but it shows there is atleast 1 sol

alpine jasper
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if 18 is the 7th power then there exist some x such that x^7 = 18

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oh then that's it?!

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there exists that x

winter bear
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yup

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show a general statement using what i showed u now

alpine jasper
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like generalize it to $x^n\equiv a(p^e)$

sterile pumiceBOT
alpine jasper
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so pick a generator of Z/p^eZ*

winter bear
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suppose $d|\phi(p^k)$, show $x^d= a (p^k)$ is solvable iff $a^{\phi(p^k)/d}=1 (p^k)$

alpine jasper
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oh hmm

sterile pumiceBOT
winter bear
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just generalize both arguments we made

alpine jasper
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i'm lost in all those symbols lmao

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gimme a moment please let me write it down

winter bear
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yeah take ur time

olive remnant
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I'm trying to figure out how many solutions there will be to a^2 \equiv 1 (mod 195) and I know the solutions to this will be the same as the solutions to the system of congruences of a^2 \equiv 1 (mod all the prime factors of 195) and I know that each of these prime factors will have two answers a= plus or minus 1 (mod some prime factor) but idk how to account for double counting solutions solutions?

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like in the case of 35 i have the system of congruences a^2 \equiv 1 (mod 5) and a^2 \equiv 1 (mod 7)

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and i'll double count 6 as a solution

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idk if that makes any sense

winter bear
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you would not double count 6 as a solution

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infact there would never be any double counting, courtesy of chinese remainder theorem

olive remnant
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fuck

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you're right

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im stupid

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idk how i thought 6 was the solution to two things

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two system of congruences

winter bear
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-6 is a solution too

olive remnant
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yeah

winter bear
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prolly messed it up there

olive remnant
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yeah

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that makes sense

alpine jasper
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is what i wrote full of garbage or

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i know we just went over an example but i'm dumb

winter bear
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so a few things

olive remnant
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thanks by the way

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i think forgot to say that

winter bear
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a^x=1 means ord(a)|x

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np

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not the other way around which is what u seem to have there

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also umm yeah the other implicition needs to be done

alpine jasper
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i can't even do one implication how can i do both lmao

winter bear
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well do this implication first then

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look back at what i did for the example

alpine jasper
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i sort of did but couldn't connect the dots retard

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let me try harder

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,rotate

sterile pumiceBOT
alpine jasper
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this makes sense?

winter bear
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yeah

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do you get an intuition for this now?

alpine jasper
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yes, but i wanna think about this part a bit more

winter bear
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aight go ahead do that, keeping in mind that a^x=1 means ord(a)|x

alpine jasper
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thank you so so much, and thank zoph also

winter bear
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np

winter bear
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(btw try continuing this to when d doesnt divide n, use gcd)

olive remnant
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so the question im trying to answer is how many solutions to a^2 \equiv 1 (mod 2^e) where e is any natural number (not zero)

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from test cases it seems that a \equiv plus or minus 1 (mod 2^e) and (mod 2^(e-1))

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so when e > 2 theres 4 solutions, else there are 2 solutions

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i started proving this but uh i realized i my proof would also show that any odd number should also work

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so i have no clue how to solve this or if my conjecture(?) is even right

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nvm got it

torn escarp
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yep it's true if you want me to check your proof or whatever

olive remnant
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i think i got it

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its just not very rigorous bc its 5:50 am

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and im too lazy

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but i just said a^2 \equiv 1 (mod 2^e) equivalent to the system of congruences a^2 \equiv 1 (mod 2^(e-1)) .... a^2 \equiv 1 (mod 2)

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which is the same as just doing a^2 \equiv 1 (mod 2^(e-1)) (which i proved in words lol)

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and then i just showed that applying this to different cases with different values for e give me what my conjecture(?) was

torn escarp
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hmm

#

what I had in mind was more like, take $a=1+2^bx$ and square it

sterile pumiceBOT
torn escarp
#

then look at which e this could be equal to 1

#

and then go back and determine what xs were allowed

olive remnant
#

ughhh

#

im sure that's better

#

i just don't know if its worth it at this point

#

im just so drained

#

does what im saying have a semblance of truth

#

so i can justify putting what i just said down

#

also i have no idea where u got that from

torn escarp
#

well you said some stuff and said "which you proved in words" and I didn't see what you said so

#

I really have nothing to go on

olive remnant
#

lol yeah :(

torn escarp
#

you sound like you just need to sleep and come back when you're fresh

#

I was terse in what I said

olive remnant
#

lol

#

aint no rest for the wicked or something like that

#

but yeah

#

thanks i appreciate the help

torn escarp
#

just try to reason why every a that's a solution is of the form 1+(2^b)*x for some b, x

#

then by squaring it you end up with some more concrete restrictions mod 2^e

olive remnant
#

well that's just cuz you can just represent it as 1 + 2^(e-1)*(2k)

#

so x = 2k and b = e-1

#

oh wait im dumb

#

still thinking

#

yeah i can't figure it out

#

ill just turn in the homework as is i guess

torn escarp
#

no you restricted x too much

#

that's not the reason why you're sorta working the opposite direction

#

$a=1+2^bx \mod 2^e$

sterile pumiceBOT
torn escarp
#

we're supposing we know nothing about b and x except that b>0 and x is an integer

#

but we know that squaring gets us 1

#

$1 = 1 + 2*2^bx +2^{2b}x^2 \mod 2^e$

sterile pumiceBOT
olive remnant
#

can't we get something larger than 1 leftover since we don't know if b is divisible by e

torn escarp
#

$0 = 2^{b+1}x ( 1+2^{b-1}x) \mod 2^e$

sterile pumiceBOT
torn escarp
#

yeah so you can start to see what are the minimum constraints to force this to be 0 mod 2^e

#

b+1=e would do it but that doesn't force x=2k

#

you just have to be careful about what b is when it's small which is where it switches over, right?

#

is this clear or do you not see where to take it from here

olive remnant
#

oh

#

i see

#

i don't understand how you started with 1 + 2^b*x

#

tho

torn escarp
#

because it's the most generic choice I could make

#

it has to be either a=0+2^b*x or a=1+2^b*x

#

for b>0

#

either it's even or odd, that's really all I'm saying

#

the 2^b is just saying there are some number of 0s (possibly none) up until this value times x

#

err written in base 2

#

sorry I implicitly assume I'm working in base 2 when thinking about stuff mod 2^e

#

since each e basically chops off a further digit, it's just cleaner

#

it's not too serious if that's confusing to you

#

either a=0+2^b*x or a=1+2^b*x

#

this is just a generic representation for some b>=1 and integer x

#

and we can assume x=1 mod 2

olive remnant
#

yeah i get that

torn escarp
#

it's just a way to force it more cleanly

olive remnant
#

ive just never seen that representation of even and odd numbers

torn escarp
#

$a= a_0+a_12+a_22^2+a_32^3+\cdots$

sterile pumiceBOT
torn escarp
#

basically looking at its base 2 expansion and then saying, there's some number of 0s

#

yeah yeah you get it I guess I forget this isn't so common to see

#

specifically working in the 2-adics is working with numbers mod 2^e for all e simultaneously

#

so I got used to that, which is where I'm kind of coming from

olive remnant
#

aight

#

sorry just thinking

#

thanks for the help

torn escarp
#

take your time I've got nothing better to do right now

olive remnant
#

im sorry i just can't

#

i wanna cry

#

i've pulled an all nighter

#

and i just can't figure it out

torn escarp
#

I'm just here to talk about math ideas, time limits and morale problems are out of my sphere sorry

alpine jasper
#

$\left(\frac{\cdot}{p}\right):(\mathbb Z/p\mathbb Z)^\times \to {\pm1}$

sterile pumiceBOT
alpine jasper
#

how would i show that this is a homomorphism..?

#

kinda lost

#

this is legendre symbol btw

light flicker
#

by proving it

alpine jasper
#

thanks i'm cured

#

..

light flicker
#

np

#

always here for you

alpine jasper
#

love you

light flicker
#

so prove it

alpine jasper
#

i guess i can try harder, i'll report back if i fail

#

actually

#

could i just map the quadratic residues to 1 and non residue to -1... uhh

#

i know that there are equal amount of them..

light flicker
#

I mean

#

That's the definition of the legendre symbol yes

#

The legendre symbol is a function from (Z/pZ)* to {\pm 1}

alpine jasper
#

$\left(\frac{ab}{p}\right)=\left(\frac{a}{p}\right)\left(\frac{b}{p}\right)$

#

this proves it..?

#

for a and b in (Z/pZ)*

sterile pumiceBOT
alpine jasper
#

is it this simple..?

#

or am i big wrong again

light flicker
#

That would prove it

#

if you know that statement is true

alpine jasper
#

ah neat

#

ireland and rosen proved that fact catthumbsup

light flicker
#

true

alpine jasper
#

very cool, thank you

light flicker
#

any time

sacred junco
#

yeah this one I dont think is easy to prove

light flicker
#

its not particularly difficult, Ireland Rosen does it in like 3 lines

sacred junco
#

thonkzoom (ab)^n = a^n b^n

sacred junco
#

lol

light flicker
#

I mean, he's not wrong

#

At least, the way Ireland Rosen proves it is to show that

#

(a/p) = a^((p-1)/2) (mod p)

#

And then you can use the fact that exponents split

wild zinc
#

most of it can be done just fine

#

but showing product of two non-residues is a residue is not as simple as the rest

#

the rest are like one line contradiction or construction

sacred junco
#

Hello

#

Can anyone help me prove Brocards theorem using induction?

#

Will be thankful to anyone who helps

torn escarp
#

,rotate

sterile pumiceBOT
torn escarp
#

the problem just wants you to just check a few cases by hand

#

proving the general statement is an unsolved problem

plain fable
#

ya it's just 8 cases (if we include 0)

sacred junco
#

Oh so i have to just put the values and check?

stoic bear
#

Yes

#

The open problem is called Brocard's problem if curious

fervent crest
#

When does the equation $$x^k\equiv a\pmod{m}$$ have a solution in $x$?

sterile pumiceBOT
torn escarp
#

when you can find $x$ for every $p|m$ such that $|x^k-a|_p < |k x^{k-1}|_p^2$

sterile pumiceBOT
fervent crest
torn escarp
#

once you have that, you can hensel lift up to the appropriate power of p and combine with the chinese remainder theorem

#

;P

fervent crest
#

Way too hardcore of a condition

#

😔

torn escarp
#

I'm partly teasing

#

well two things

#

the p-adic absolute value part can be reworked into just modular arithmetic if you want to see that part,

#

$$f(x)\equiv 0 \mod p^{2a+1}$$ $$f'(x) \equiv 0 \mod p^a$$ $$f'(x) \not \equiv 0 \mod p^{a+1}$$

sterile pumiceBOT
torn escarp
#

usually people just show the a=0 case for Hensel lifting but this is the more general circumstance when f'(x)=0 mod p

#

idk hopefully that's not too scary as a requirement

#

but in some sense I didn't really answer the original question either, I just pushed the answer back to looking at x^k=a mod individual small powers of primes

fervent crest
#

🙃 Take in mind that I don't know a lot about p-adic

torn escarp
#

ok ok back up have you seen this condition for solving for x for f(x)=0 mod p^k

#

if you satisfy $f(x) \equiv 0 \mod p$ and $f'(x) \not \equiv 0 \mod p$ then you can lift?

sterile pumiceBOT
fervent crest
#

Not the |x^k-a|_p condition you posted above

#

Oh

#

Yeah you've showed me this

#

I think

torn escarp
#

yeah

fervent crest
#

It was in the hensel's lemma

torn escarp
#

I didn't want to scare you too bad so I only showed you this case when f'(x) != 0

#

plug in a=0 and you see this second condition is f'(x)=0 mod 1 which is trivially true

#

it collapses back down to that, this is just so that when you look at your f(x)=x^k-a (different a from my powers whoops bad choice)

#

your f'(x) = kx^{k-1} might have k divisible by some power of p in common with m

#

which can mess things up

#

take gcd(k,m)=1 and your problem becomes simpler

#

now you really are just looking at x^k=a mod p in all circumstances

fervent crest
#

Oh

#

Well if gcd(k,m)=1 then I have an elementary method to solve this equation

#

Idk if this guarantee existence tho

torn escarp
#

yeah you need to start talking about the phi function / fermat's little theorem or something

fervent crest
#

Oh yeah

#

The condition isn't gcd(k,m)=1

#

my bad

#

it was gcd(phi(m),k)=1

#

Then you write u*k-v*phi(m)=1

#

somehow make u,v≥0

#

then

#

$a^u\equiv\br{x^k}^u\equiv x^{1+v\cdot\varphi(m)}\equiv x\pmod{m}$

sterile pumiceBOT
fervent crest
#

But like

#

I don't think this shows the solution exist

#

Oh wait nvm it does

#

I'm dumb xD

#

But still what if gcd(phi(m),k)≠1

torn escarp
#

I stepped away for a bit

#

hmm what you wrote looks backwards to me, seems like you're assuming a=x^k

fervent crest
#

?

#

So I assumed gcd(phi(m),k)=1, then write u*k-v*phi(m)=1 where u,v≥1, then you have that x=a^u is a solution

torn escarp
#

your first equals sign, how do you make this substitution:

#

$a^u\equiv (x^k)^u \pmod{m}$

sterile pumiceBOT
torn escarp
#

aren't you assuming a=x^k here?

fervent crest
#

yeah

#

i realized that

#

but it's also really easy to show that a^u is a solution

#

$\br{a^u}^k \equiv a^{uk}\equiv a^{1+v\cdot\varphi(m)}\equiv a\pmod{m}$

sterile pumiceBOT
torn escarp
#

oh ok

torn escarp
#

I guess the next thing is if gcd(k,phi(m))>1 to split off the offending powers of primes from m so that we have the simple cases taken care of

#

and then specifically look at gcd(k, p^{n-1}(p-1)) > 1

fervent crest
#

Hm ok

torn escarp
#

kind of distracted but I got a handful of results for some cases

#

if k isn't divisible by p, then you just need to have x^k=a mod p

#

i k is divisible by p, and p>2 then you need the numerator of (a-1)/k divisible by p once

#

if k is divisible by p and p=2 then you need the numerator of (a-1)/k divisible by p twice

#

that's enough to guarantee you have a solution but there could be some weaker conditions I'm pretty sure

#

@fervent crest tell me if these are obvious or not I have tunnel vision

fervent crest
#

I'm not sure what you meant by offending powers of prime

torn escarp
#

oh

#

gcd(k, phi(m)) > 1

#

so you factor m into two parts m=r*s such that gcd(k, phi(r))=1 and gcd(k, phi(s))>1

#

then we look only at the powers of primes in s

#

and look at just these cases for the sake of gluing them back together with the chinese remainder theorem

#

because phi is multiplicative and I'm picking gcd(r,s)=1 for m=r*s

fervent crest
#

Ok

torn escarp
#

gcd(k, phi(m)) = gcd(k, phi(r)*phi(s)) but

#

the idea was to pick them such that this splits as well, that's what I meant by offending

fervent crest
#

Hmmm ok

inland hawk
#

has anyone used the chinese remainder theorem before

#

to solve a system of congruence

light flicker
#

What are you confused about?

inland hawk
#

i got an answer but it doesnt check out with the second congruence 3(mod 5)

light flicker
#

Maybe show your work?

inland hawk
#

sorry couldn’t get a good picture

#

I got 662

light flicker
#

Ah yeah

#

check your b2

inland hawk
#

yeah I get 2(mod 5) when I check it so ik that’s wrong

light flicker
#

Yeah, so check your calculations for b2 again

inland hawk
#

i think I’m blind or just missing something lol

#

I keep getting b2=2

light flicker
#

what's 252 * 2 (mod 5)?

inland hawk
#

4

light flicker
#

but what do you need it to be

inland hawk
#

1

light flicker
#

so what do you need b2 to be instead

inland hawk
#

3

light flicker
#

yes

inland hawk
#

oh ok lol

light flicker
#

And I mean, you even wrote it down

#

that 2 * 3 = 1 (mod 5)

inland hawk
#

thx im a silly goose

#

yeah I was just kinda following another example without even thinking

sacred junco
#

I noticed that
15/154 = 9.7%
10/149 = 6.7%
is that a coincidence?

#

15-1/3 * 15 = 10
9-1/3 * 9 = 6

stoic bear
#

?

torn escarp
#

there's no pattern here

sacred junco
#

likewise 20/159 = 12.6%

torn escarp
#

what

sacred junco
#

10/149 = 6.7%
15/154 = 9.7%
20/159 = 12.6%
25/164 = 15.2%
30/169 = 17.7%
35/174 = 20.1%

it seems to me like you can roughly approximate the %

#

10/149 = 6.7
12/151 = 7.9
14/153 = 9.1
16/155 = 10.3

#

obv few jumps like 15.2 to 17.7 and you'd be off by quite a bit

#

but it's still better than nothing

winter bear
#

im guessing your claim is that the approximate increase is by 3% at each turn?

sacred junco
#

@winter bear with 5 yeah

#

with 2 it seems to be 1.2

winter bear
#

$\dfrac{n}{x+n} = \dfrac{n}{x} \dfrac{1}{1+\dfrac{n}{x}} = \dfrac{n}{x}-\dfrac{n^2}{x^2}+\mathcal{O}((n/x)^3)$

sterile pumiceBOT
winter bear
#

with x>>n

#

this approximation should show why its increasing at about a 3% rate when ur numbers are small enough

sacred junco
#

what's that letter at the end

winter bear
#

big O notation

sacred junco
#

when do you learn it

winter bear
#

just google it, the defination is p easy to understand

sacred junco
#

also what does the >> mean

winter bear
#

much greater than

sacred junco
#

thanks

winter bear
#

im using some taylor series approximation, if your familiar with those

#

cause thats what your problem essentially breaks down to

#

you are seeing the linear term in the expansion dominate for small values of n

sacred junco
#

thanks for that, I'll try to read some wiki pages on it

rigid loom
#

Any books u would recommend for number theory?

light flicker
#

How much math have you studied?

#

@rigid loom

rigid loom
#

I had maths in my alevels but now am in first year of uni

#

Having trouble with modular arithmetic and stuff

light flicker
#

hm, you could look at Silverman's A Friendly Introduction to Number Theory

#

But, if you just want to understand the stuff you're struggling with, you can just ask here, or there are videos online

rigid loom
#

More of a book guy tbh

light flicker
#

Yeah I mean, trying to read a book just to figure out modular arithmetic probably won't be the most efficient

#

As in, this book goes over some unrelated stuff before talking about modular arithmetic and you might not be able to skip that stuff

rigid loom
#

Ahh i see alright I’ll look into this stuff tbh and if encounter any problem will get back to u

light flicker
#

I could always just try explaining stuff to you too

rigid loom
#

Thanks budeeveeKawaii

alpine jasper
#

why is the flipping rule allowed with (1001/9907) even tho 1001 is not a prime?

#

i know that the Legendre smbol is defined for (a/p) a need not be a prime

#

but what confuses me is why can i flip without a being a prime too since the law says q and p both ood prime

#

pls come to rescue zoph papa @light flicker

supple furnace
#

The situation for the Jacobi symbol is a bit different. Jacobi symbols don’t always detect QRs properly, but they will always detect QNRs (if the Jacobi symbol is -1, you have a nonresidue).

alpine jasper
#

right, i understand that, but not what i'm asking, sorry for my shit wording above

#

the one on the left is a Legendre symbol, yes?

light flicker
#

Jacobi symbols are equal to legendre symbols when the "denominator" is prime

alpine jasper
#

hmm alright

#

but how does that allow the flip.. in the first step?

light flicker
#

quadratic reciprocity

alpine jasper
#

but that requires both top and bottom to be primes tho...?

light flicker
#

Look at property 6 on that wiki page

supple furnace
#

Since the Jacobi symbol matches with Legendre symbol in prime case, it’s always right

#

It has the do with the number of QRs modulo n in general

alpine jasper
#

oh huh, so since the LHR is both Legendre and Jacobi, i just treat it like Jacobi and get 1 in the end

supple furnace
#

The Jacobi symbol takes permutation signs so it is half +1 and half -1 always while quadratic residues mod composite moduli are much rarer in general

#

Yeah

alpine jasper
#

i see

#

cool stuff

rigid loom
#

Pub is that u???

alpine jasper
#

ain't that a coincidence lmao

rigid loom
#

Wtf

#

Oh well

sleek cove
#

fermats little theorem is basically useless with p=2 right

torn escarp
#

lol

#

yeah I guess so

fervent crest
#

I mean if you have to state it in that way

sleek cove
#

im just trying to solve a problem using flt but it would rely on 2 actually doing something

#

now i have to do extra work angerysad

light flicker
#

happens very often in nt that 2 is a special cas

alpine jasper
#

let's kick 2 out of the prime club

#

fundamental theorem of arithmetic is only for chad odd numbers

rigid loom
sacred junco
#

228

rigid loom
#

Can u explain

#

I am@weak at these

random hill
#

What remainder is left after dividing by that number

alpine jasper
#

$232\equiv 4(\text{mod }x)$ is same as saying\
$232=4+xn$ for some integer $n$

sterile pumiceBOT
alpine jasper
#

@rigid loom

sterile pumiceBOT
rigid loom
#

Ohhh

alpine jasper
#

~~yeah alright throw him some ideals lmaothonkeyes ~~

winter bear
#

always generalize stuff in Z to rings smh

alpine jasper
#

i'm not far enough in nt to see why that'll be very useful

#

nice new pfp btw, it was one of my favs

winter bear
#

thx

sleek cove
#

in proving a^(p^k) = a mod p, induction is fine, right

winter bear
#

mhm

sleek cove
sleek cove
#

can someone tell me if this logic is flawed

#

i dont want to have to do induction again, but this just seems kinda cheeky

somber drum
#

I got an assignment to solve a problem using newtons method for multiple variables

#

sat all day coding in matlab and i thought it would work but didnt

#

and when I debug I notice that for some reason f1 and f2 become the same value after the first iteration has passed

#
function NewtMultiVar
        function newtMulti = newtMulti(xk)
        u = xk(1);
        v = xk(2);
        function f1 = f1()
           f1 = ((93-u)^2)-((63-v)^2)-(55.1^2);
           disp(['f1 : ',num2str(f1)])
        end
        function f2 = f2()
           f2 = ((6-u)^2)-((16-v)^2)-(46.2^2);
           disp(['f2 : ',num2str(f2)])
        end
        function df1u = df1u()
           df1u = -2*(93-u);
        end
        function df1v = df1v()
           df1v = 2*(63-v);
        end
        function df2u = df2u()
           df2u = -2*(6-u);
        end
        function df2v = df2v()
           df2v = 2*(16-v);
        end
        function S = S()
            s1 = sym('s1');
            s2 = sym('s2');
            
            E = [(df1u()*s1)+(df1v()*s2) == -f1(), (df2u()*s1)+(df2v()*s2)==-f2()];

            X = solve(E,s1,s2);
            S = [X.s1 X.s2];
            S = double(S);
            
        end
        newtMulti = xk + S(); 
    end
#

The disp is just to debug

#

I dont get what im doing wrong, (in the body i call this iteratively but thats irrelevant, this is the function that calculates)

#

After one iteration f1 and f2 turn out the same

#

and my results just spasm out of control

#

I'm going insane, spent entire day on this and Im pulling my hair out, defeated

#

Ill give my first born child to anyone who can help, i'm at a total loss..

#

<@&286206848099549185> if possible

sleek cove
#

can someone help me with this problem, i've broken it into modulo 2 and 5, giving

#

x = 1 mod 2, which is obvious, but idk what to do for modulo 5

#

i know that (x^4)^250 = x^1000, but dont know how to apply FLT

#

do i assume that x has an inverse and is a unit? or what...

#

nvm

fervent crest
#

I got 1, 3, 7, 9

sleek cove
#

yes

#

x=0 mod 5 gives contradiction, then just have to solve the 4 other cong. when x = 1, 2, 3, 4 mod 5

fervent crest
#

You can do it with euler’s theorem

#

i.e. x^(φ(n))=1 (mod n) if x is coprime with n, where φ(n) is the euler totient function

#

Euler totient of 10 is 4, therefore automatically 1, 3, 7, 9 are solutions

#

Then for 0, 2, 4, 6, 8, when you raise them to arbitrary power it’s always even therefore never 1

#

For 5, when you raise that to any power it’s always congruent to 0 or 5 (actually it should always be 5), so it’s also never 1

sleek cove
#

we havent got to that yet

#

looks fancy

sacred junco
#

can someone try deciphering this word salad

stoic bear
#

What is a word salad?

sacred junco
#

I imagine the lecturer was wanking himself off while writing this thinking hes a genius

#

it's clearly not parsable

stoic bear
#

I think the lecturer just wanted to direct you to other related topics to explore

sacred junco
#

this is supposed to be a solvable question

stoic bear
#

Also, it isn't really unreadable, at least it isn't a mess of symbols

sacred junco
#

okay please enlighten me

stoic bear
#

On?

sacred junco
#

what is the question asking?

stoic bear
#

The first sentence?

sacred junco
#

yes, the question

stoic bear
#

The 1st sentence is the question

#

"show that..."

sacred junco
#

can you put it in words that humans understand?

#

how is it "producing" said sequence

winter bear
#

the wording is p bad on this

#

but i think he means to show that Z/pZ* is cyclic

sacred junco
#

he's a fucking charlatan

stoic bear
#

I don't get it upon further examination, sorry downtown

random sand
#

Idk man

winter bear
#

i.e show that the sequence {a^n} for some generator is lenght n-1?

#

p-1

#

no

random sand
#

Probably equivalent to that anyway

winter bear
#

p-1

#

Z/pZ* is p-1

#

actually

#

i think?

random sand
#

This is weird

winter bear
#

i mean the proof is p involved anyhow

#

the one i know uses mobius inversions

random sand
#

Is it showing that a^n is periodic (as a function of n)?

sacred junco
#

such a fucking fraud

random sand
#

I’m out I can’t read it

winter bear
#

yeah idk this is hard af to parse

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its like he forgot a sentence

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or smth lol

stoic bear
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Sorry, downtown

random sand
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I can’t give a definitive answer as to what he wants

winter bear
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can we see the whole problemset @sacred junco

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maybe theres some context in other problems?

sacred junco
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there isn't

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it's all unrelated

winter bear
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but in any event, your best bet is prolly to just email him

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rip

sacred junco
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I'd rather just call it out publicly

winter bear
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eh bad move, man controls your grade, just give him the feedback that he made a mistake

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everyone makes mistakes lol, doesnt mean they should be shamed for it right

sacred junco
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no it's pretty consistent

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he's a charlatan who thinks he knows what he's talking about

light flicker
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wait what do you guys not understand lmao

sacred junco
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he's writing a book that is riddled with mistakes

light flicker
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its just asking you to show there's always a primitive root

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and not even for F_{p^n}, its just F_p

winter bear
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i mean thats what i guessed, but it really doesnt say that?

sacred junco
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I hate this guy, he's a total pseudointellectual

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it's a computer science course and this is his attempt to teach mathematics

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but he's failing since he doesn't know any

light flicker
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I mean, it says to show that if you take a primitive root, then it'll produce such a sequence

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I mean, its pretty clear he does know mathematics lmao, this is fine

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I mean, he might be a terrible teacher, but thats different

winter bear
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nah the wording is still p bad, most of us guessed he meant show its cyclic but still

sacred junco
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you think this is good evidence that he knows mathematics? a word salad with words he doesn't even understand?

random sand
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I can’t read this right, but I wouldn’t go so far as to say he’s a charlatan

winter bear
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i mean he doesnt use any particularly big words

light flicker
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It's not a word salad lmao, it makes mathematical sense

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And it's correct math as well

winter bear
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my problem is that he just didnt phrase it well enough

light flicker
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I mean, maybe he just copied it from somewhere and he doesn't know the stuff, but its correct mathematically

sacred junco
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his construction of the real numbers was $\mathbb{R} = \overline{\mathbb{Q}} \cup \mathbb{Q}$

sterile pumiceBOT
random sand
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Doesn’t seem like he hides behind terminology, it’s just I can’t read it

sacred junco
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in his great book that we all get to read

winter bear
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yeah rifts, its like missing a sentence i feel like

sacred junco
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I'm a math major taking his course

winter bear
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i mean i think by that he means like the completion of Q?

serene crag
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Lol it sounds like he is confused. But maybe the theorem he's trying to get at is that for every prime, there exists a primitive root.

sacred junco
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no, he was saying that the real numbers are the rational numbers with the rationals

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which is blatantly wrong

light flicker
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You mean the irrationals?

sacred junco
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yeah

light flicker
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Because that's correct?

sacred junco
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...

light flicker
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lmao

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Irrationals are literally defined as the real numbers that aren't rational

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So maybe this might be circular, but its definitely not wrong

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And there are plenty of other ways to define the irrationals

random sand
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I don’t see what exactly would make that wrong per se, tho idk what kinda closure that overline refers to

serene crag
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$\mathbb{Q}$ is the algebraic numbers, not the irrationals.

sterile pumiceBOT
light flicker
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if you mean \bar{\bQ}

serene crag
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sorry. meant to put a bar.

light flicker
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this notation is used for both

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just because algebraic numbers come up more, you see that used more frequently

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Anyways, its clear that he meant irrational here and thats not the confusion

random sand
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Like, is the bar for a metric closure, algebraic, etc?

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it’s annoying hoe stuff gets reused

light flicker
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set complement

random sand
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ew who tf uses bar for that

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disgusting

light flicker
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@sacred junco so uh, have you figured out why this is right yet

random sand
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It’s right but wow I don’t like it

light flicker
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I mean, its probably the most common notation for denoting irrationals

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not that people do that often but

winter bear
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bar for irrational is ew

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or complement*

sacred junco
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@light flicker nah

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I haven't tried

light flicker
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Anyways, your professor is right, it's not "blatantly wrong"

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Whether its a good way of defining the reals is another question

sacred junco
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I still think he's a charlatan

light flicker
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Idk, you just seem mad at this guy and are trying to blame him for everything, including you not understanding any of this

random sand
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I don’t know enough to reliably talk about his skill & knowledge, but I certainly can’t say that he is an imbecile

sacred junco
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nah I just don't have time to decipher this stupid question

random sand
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Clearly he knows something

winter bear
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yeah, as i said he prolly just forgot to put a sentence into that question

light flicker
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I mean again, its not stupid just because you can't understand it

sacred junco
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@light flicker that's not the reason I think it's stupid

light flicker
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Then why do you think its stupid

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It's a fine exercise that introduces important number theoretical ideas

sacred junco
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because it's phrased in such a way that no reasonable person should be expected to be capable of being able to decipher it

light flicker
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I mean, I understood it fine

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If you understand what primitive roots are and fermat's little theorem is, you should be able to understand this

random sand
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I couldn’t decipher it but I am an imbecile

sacred junco
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@light flicker could you rephrase the question?

light flicker
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Do you understand what Fermat's little thoerem says?

sleek cove
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theres a lot of smart ppl here PepoG

sacred junco
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yes

light flicker
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And what does it say

sacred junco
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if $a$ is not divisible by prime $p$ then $a^{p-1} = 1 \pmod p$

sterile pumiceBOT
light flicker
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Okay, and do you understand what primitive roots are

sacred junco
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no

light flicker
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Then you should try to understand that?

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Not knowing one of the terms in the problem will obviously make the problem confusing

sacred junco
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where is "primitive root" a term in the problem

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for the record there is no expectation of understanding primitive roots before attempting this question

light flicker
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"The \alpha here is known as a primitive root"

sacred junco
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ok i got that

serene crag
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Why does he say there's a "unique" sequence? Different primitive roots can give different sequences.

light flicker
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Yeah, it means unique as in choosing different alpha will give you different sequences

sacred junco
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okay so I choose alpha = 5, what is the sequence given

serene crag
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As an example, if p = 5, 2 and 3 are primitive roots because they give 2, 4, 3, 1 and 3, 4, 2, 1 resp., but 4 is not because it goes 4, 1, 4, 1, etc.

light flicker
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The sequence here is the powers of \alpha

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for when p = 5, \alpha = 2, the sequence is 2, 2^2, 2^3, 2^4

sacred junco
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up to p-1

light flicker
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which you can see reduces (mod 5) to be what he described

sacred junco
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so how do I prove the uniqueness?

serene crag
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This is also a pretty hard problem to do without any hints. An outline is, you define the order of an element z to be the least d for which $z^d = 1$. A primitive root is one whose order is p-1. Now you can show that the order must divide p-1 by FLiT. Then you show that for each d, there are at most phi(d) guys with order d. Then you use the fact that every element has an order and the fact that $\sum_{d|p-1}\phi(d) = p-1$ to show that this implies that in fact, there are $\phi(d)$ guys with order $d$. In particular, there are $\phi(p-1)$ primitive roots.

sterile pumiceBOT
serene crag
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I still don't understand what "uniqueness" really means here.

sacred junco
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I'm guessing it is uniqueness for the same p

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@light flicker could you state the claim in precise terms?

light flicker
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Sorry, I was in the shower but

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I'd write it as

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Show that for any prime $p$, there exists primitive roots mod $p$. In other words, there exist numbers $\alpha$ such that $\alpha^{p - 1} = 1 \pmod p$ and $\alpha^n \neq 1 \pmod p$ whenever $0 < n < p-1$.