#discrete-math
1 messages · Page 32 of 1
U came up with one claim
Your two claims together make up one claim equivalent to this. However, each of your claims on their own aren't equivalent to this.
So how about 2 cases : 1. X is in A (complement) and x is in B (complement) 2. X is in A and x is in B
Sure tbh I'm really lost now on where you're going with this
Forget the two claims
This is my claim
And I need cases to prove this claim
Do I need cases at all for this proof? Can I just prove it without any cases? If not how many cases would I need
then do this^
you would definitely need at least two cases right because you need to show the right hand side is a subset of the left hand side and the other way around
Also note that A intersect B is precisely just A
since A is a subset of B
set theory proofs are never fun lol
Idek where to start
but it's good practice
😭
well suppose X is in B and not in A. for the first case. Then suppose x is in A for the second case
that would be my outline of the proof, the rest is up to you
What’s wrong with the cases I had?
first off, case 1 seems really confusing because if x is not in B, then surely x is not in A, so redunant at the very least. Case 2, is also very redundant because if x is in A, then x is in B because we assumed in the first place that A is a subset of B
I see
to be more precise I would just stick to doing this
So for these two cases, what would be the “WTS” part ?
for the first one, it would be claim 1 here^. For my second assumption it would be claim 2 here^
because either "x is in A and therefore B" or "x is not in A but is in B" those are really the two meaningful cases
I'm gonna be afk for a bit getting lunch and such...but if u need any help in the meantime ping the Helpers
i have to do this exact proof too lol
for this claim, I am getting stuck at $pk = qf, k,f\in\mathbb{Z}$
42
They are relatively prime so I know i should do something with that from here on
but not sure how
Did you figure out how to solve it?
I havent done it yet
Wait you’re saying I should use these 2 as my cases for the claim?
Didn’t you say it’s not right
this is what I was replying too^
read this^
After telling you your two claims weren't on their own equivalent to the statement with the equality, I later got the impression that you were wondering how to prove the equality. In which case, your two claims "cases" are a good way to go for a proof.
For example: for sets A and B....."A is a subset of B" isn't equivalent to saying "the set A equals B". However saying "A is a subset of B and B is a subset of A" is equivalent to saying "the sets A and B are equal"
Ok so I’m gonna use these two cases to prove my claim in that case (no pun intended)
yes proving those two would prove this^
Ok, now I need a starting point for case 1 in set notation
Can we use small a instead of x
no
Ty
logician
There's your first statement
Ok
So $a\notin(A-B)\cup(B-A)$.
logician
Right
so $a\notin A-B$ and $a\notin B-A$.
logician
Is there a set notation for “and”
intersect
Ok so upside down U
$\cap$
logician
yup
Ok
let's resume from here
Sure
$a\notin A-B$ implies $a\notin A$ since $A\subseteq B$.
logician
recall $A-B=A-(A\cap B)$.
I’m thinking if that makes sense in my head
logician
Ok yea
$A\cap B=A$ since $A\subseteq B$.
logician
now looking at this again
The above is part of the proof?
this^?
It’s not letting me go to the message
This one
yes I mean this could really be the very first statement above both cases, but yea helpful to put it in your proof
okay so $a\notin B-A$, what does that mean?
logician
I mean like if i place this step right after the “a is not in A - B implies” line part would it make sense
i honestly won't know until I see your final draft of your proof, right now we're going through the proof while I'm giving you pointers on why things are true (not all of these pointers need to be in the proof)
Oh ok
milly let's get back to this^
So this means a is not in B - A
Like if we have an int a, the int a would not be in set B
right
I would say this implies $a\in\overline B\cup(A\cap B)$.
logician
That would be true
because $a\notin B-A$ implies $a\in\overline B$ ($a$ wasn't in $B$ in the first place) or $a\in A\cap B$ ($a$ was thrown out of the set $B$ when we calculated $B-A$).
logician
Hmm
you following?
I get that a is in the complement of B but I don’t get how a is in A intersection B
$B-A$ means take the set $B$ and delete the elements of $B$ that are also in $A$.
logician
Recall $A\cap B=A$
logician
I think I get it now
okay great so that's the end of case 1
we just showed $\overline{(A-B)\cup(B-A)}\subseteq \overline{B}\cup(A\cap B)$
logician
yes I'll leave that up to you right after I go over one piece we should elaborate more on from case 1
I would say case 2 is similar to case 1
Is it possible that case 2 proof is backwards to what we did in case 1?
yes, saying $a$ is in the LHS of the equation, is the same as saying $a$ is not in the union of $A-B$ and $B-A$. Meaning $a\notin B-A$ and $a\notin A-B$.
logician
$a\notin B-A$ is how we got the first case complete
logician
Case 2: let $a\in \overline{B}\cup(A\cap B)$
logician
Right
so $a$ is notin $B$ or $a$ is in $A$
logician
if $a$ is not in $B$, it definitely won't be in $B-A$
logician
The second step would be that a is not in B
But I don’t get how to write the other half of that
if $a\in A$, then $a$ won't be in $A-B$ because $a$ is in $B$ as well
logician
other half of what?
The part after B complement
hence $a\in\overline{(A-B)\cup(B-A)}
.
what part about this is confusing? where are u getting lost?
do you agree that means $a$ not in B?
logician
or a is in A intersec B which is A
I’m getting a bit confused
Too many things going on lol
For case 2, I have one step written down
And I’m a bit confused about step 2
Are you explaining what the proof will do or is this the title of case 2
This is not the title lol
It follows from this^
I get that a is not in B because of the complement
Alright so a is in B complement or A intersect B, right^^^
Yes
Sure
Yes
Great so in this case a is in the set on the LHS of the equation we’re trying to prove
Cuz it’s not in A-B and not in B-A
No we’re trying to show a is in the set on the LHS of the equation we’re trying to prove
We already proved the other direction….
We already did that
No but the case 2 is switched
Read this
What do u mean switched? We need to show the RHS is a subset of the LHS
So we did half of case 2
Now the other half of case 2 is if a was in A intersect B
U following???
Yes
Yes
So a notin A-B since everything in A is in B and would therefore get thrown out
Following?
Yea
Great a in A also means a notin B-A because it would get thrown out of B
So we just showed the RHS is a subset of the LHS
Proof is complete
That’s fine. Clean it up I walked you through the proof so as you’re writing up ur final draft just look back here
For case 1 I think it’s good but case 2 I’m missing steps
And it’s a bit for confusing then case 1
Look at our chats lol I don’t wanna say the same thing 3times again when u can just scroll up
Uh don’t do that
Clean it up
And then it’s complete
While it’s still fresh in ur mind
It’s draining …
There isn’t a set notation for “also” is there?
Depends on the context…if a is in A and also in B then use that upside down U
For case 2 I have it as, a is in A which also means a is not in (B - A)
There’s set notation for and or and not
That should say if a is in A….
Because the other option was a was in B complement
K I’m gonna do some hw and see if anyone else needs some help
I have the good copy (I think) for case 1
Lmk when both cases are done
Can we work on them one at a time to prevent confusion
I wanna see a complete proof
Besides case 1 is more clear to me atm
Sure post a pic of what u got
type what you have
That’s too hard
ok outline what you got
Shall I try dming it ?
sure
you should be able to upload pics here so not sure why that's not happening but yea we can try
<@&286206848099549185> Could someone help Milly. I've outlined a proof for Milly, we've been at this for 3+ hours because seems to be having a hard time following along...I don't wanna give the answers outright, hoping someone else here can explain it in a way that may help trigger his mathematical thinking.
Pls gives me a hint on this. I think we are using Pigeon here but I don't know where to apply it.
A domino tiling problem on 2 x n arrays:
https://stackoverflow.com/questions/37744971/filling-up-2-x-n-array-with-dominos
This gives us the fibonacci sequence.
Can someone explain to me, why we can simply add the probabilities?
So when we tile vertically -> f(n-1) probabilities or options
tile horizontally -> f(n-2) options.
I get that, but why can we simply add these? Is there a proof behind this? How does it work?
can anyone help me understand the third and the fourth example?
i want to say that there are no such functions but im sure that im missing something
Hi guys, is the only way I can prove a and b using a truth table or is there another way?
well truth table is by far the easiest way
and in the end every proof will be equivalent to the truth table
for a) anyway. for b) you can use a) assuming you already know stuff about 'and'
Thanks alot @little prism
In a workplace is a pool of 8 printers. One of the printers stops working. In how many possible ways can 19 print outs in the que be divided between the 7 functional printers if:
a) at least 1 printjob goes to each printer
b) at most 4 prints go to every printer
I got 177100 on a), is that correct?
I'm not sure how to solve b)
I have a question regarding proof by induction (the theorem i'm proofing is about the adequacy statement for logic which makes use of trees/semantic tableaux). Basically what it comes down to is, imagine we have tested a property for the base case so it holds k = 0 for example. And then we assume that it holds for k, and I want to prove it for k+1. Can I then say that the property holds for everytihng smaller than k and use that to prove it for k+1 or not?
Hi guys, for this question, does my answer look correct, and how do I find the weakest preconditions, is it the top precondition {v<-1 or w>0}
that is called strong induction and is fine, yes
Okay thank you very much!!!!!
how to prove $p∨q, q∨r, p->¬r |- q$ using natural deduction
thatFirla
What I am struggling with is having two disjunctions and I don't know what to do? Because won't I have like 5 outcomes and all need to be true?
Hi
What number is it?
What number would -x be?
Hmm
By substitution
y = 4. There exists only one x which satisfies the given formula
Then x = -4
4 = -(-4)
Is this right?
@pearl fern

@final cliff
sorry for pinging but I am so curious about this
What are you trying to figure out here?
What number would x be in the formula y = -x if substitute y by 4
And, also,
What is the notation for equinumerous sets…
I will kiss you if you clean my doubts
You're trying to compare the set of positive "number" and the set of negative "numbers" right? @thorny hollow
No, it is about the print I send above there
The formula
y = -x
It is a bijection
, yeah
Substitute the variable y by 4
4 = -x
What will x be
-4?
"It can be seen at once that this function maps for instance, the set of all positive numbers onto the set of all negative numbers ..."
This is from your image.
Yeahthen yes I am
A formula by itself is not a bijection fwiw. Bijections are functions from one set to another that satisfy extra properties.
Here one of your sets is the positive real numbers and the other is the positive negative numbers.
So, you can define a function $f:\mathbb{R}^+\to\mathbb{R}^-$ such that $f(x)=-x$ for any $x\in\mathbb{R}^+$.
DootDooter
Pretty sure this is the function they're talking about is all.
It's the same function I mentioned earlier.
For any x in the domain you map x to -x.
Not sure how much my notation makes sense to you.
$\mathbb{R}^+$ is the set of positive real numbers. $\mathbb{R}^-$ is the set of negative real numbers. $f:A\to B$ means f is a function with domain A and codomain B. $x\in A$ means x is an element of the set A.
DootDooter
This I understand
If you want to show my f is a bijection it just takes some arithmetic and basic facts about the sets above.
If y is a negative real number, then -x is a positive real number. Notice f(-x)=--x=x.
This shows f is a surjection onto the negative real numbers.
If x and y are positive real numbers and f(x)=f(y), then -x=-y by definition of f, so x=y follows by properties of real numbers.
This shows f is an injective function.
So, f is a bijection
You see what I mean @thorny hollow ?
I was looking at this question, and wondering why the answer wouldn't be $\forall_x \exists_y (P(x,y)\wedge \neg Q(x,y))$
bossysine
Hi everyone. I colored the successive neighbours of a tile in an aperiodic hat-tiling and it looks like it produces hexagons. Is there a way to prove that ?
That is correct actually - the only difference is they wrote “not (not P or Q)” and you wrote “P and not Q” but since those are equivalent, the two answers are the same
thanks
Hi guys I have a question
proof by contradiction basically means we assume the negation of a statement to be true? And we try to find a single contradiction from one of its premises to disprove that?
basically if we found out that a premise is false then we can conclude that the negation of a statement is actually false from just this singular contradiction
Therefore, if it this statement is false then the negation of the statement should be true instead?
I hope someone could help me stress-test my understanding of the concept, thank you!
I think you get the main idea. Proof by contradiction is just a technique to show the negation of a particular statement holds.
The mechanics of actually doing that are to assume the statement is true, do some calculations and inferences, derive some contradiction, and then to conclude the negation of your original statement must be true instead based on the contradiction.
You basically said all of this already, just phrased a little differently.
man i hate discrete math, especially cuz the teacher mad ass
can someone help me understand what this means?
think about it by replacing y with x^2
y must be be the square of x in order for (x,y) to be in R.
that is only (x,x^2) is in R
@tidal prawn
Hi. Can anyone explain where they got the 4 from? is it because its a factor of 8?
a divides b
That is, there is an integer k such that b = a*k
oh ok thx
I know the answer is b and e, but I was wondering if c and a are the same thing in this statement?
is AxEy == EyAx?
Yeah that's pretty much it. Odd number = can be written as 2k+1, but also means can be written as 4m+1 or 4m+3 (useful to keep in mind for proofs like this!)
I know AxEy ~= ExAy but I'm not sure if this is logically equivalent
oh
I looked it up and AxEy ~= EyAx
but I dont see how they would be different in the question
I think statement c says for all children of an existing parent?
and statement a says there exists a parent that for all children
which says two different things
I'm not sure if I translated that right though, could someone let me know if the quantifiers were translated correctly
Ok thank you
yeah im not sure if c is translated right and if it is then isn't that equivalent to a?
a) is "there exists x, such that for all y, y is the child of x, and y works at a grocery store"
(idk why W(x) became W(y))
I think that might be just my teachers mistake I had a couple of problems on my homework where she would accidently switch variables
c) is "for all y, there exists x such that y is a child of x and y works at a grocery store"
So for a), there would be someone who's a parent of all the children. For c), each child has a parent (but this needs not be the same parent)
oh ok
+whatever about working, but idk that it's right as writen. In either case, this part alone shows they are different
Is this an acceptable proof or is there any steps I skipped in the parts after we must show that
looks good to me! I'm convinced!
👍 thanks for the confirmation
I wrote a proof for a problem I assigned myself...was wondering if my proof is convincing to anyone else....I'm convinced but yea wondering if u guys are
pls ping me with whether or not ur convinced....I'm gonna grab some dinner rq and will prolly be helping other ppl in other channels
``$|S| = \infty$" is very weird notation
bee [it/its]
i agree lol
I wouldn't call it complete
also it kind of feels like you just skipped over most of the proof?
I did
LMFAO
no but my thinking is
like everything you said is true but you kind of just make one true statement and then jump to "so S is infinite, this is a contradiction"
Whatever your thinking is, it should be part of the proof if relevant, no?
since s_i is arbitrary, then for s_l, there's something in S greater than that etc. Since s_j is also in S, then there's something in S less than that etc...
I should prolly right that^
damn thats difficult
That doesn't really fully show it's infinite though
I'm using that statement I proved recursively
I proved that u give me any element of S, I can find something in S smaller than it
u give me that "smaller" thing, I can find....
similarly for the greater elements
Like yes that would be correct. But if I was grading it I would give you like 50%
Ryx
ok tanks
Yea I mean definitely more detail would be helpful to add in my proof
LOL
Ryx also agrees with me that it's correct so I'm taking that win. But always good to hear other ppl's opinions on math work. That's how we get better, right?
I would either prove it inductively, or do something along the lines of that you did to show (and ofc include this part in the proof) that if no max existed, you could then create an infinite, strictly increasing sequence of elements
I agree
I want to be good at this proof stuff even though I am not lol
yup i'm implicitly using induction
Aight thanks for the tips Ryx, much appreciated
(and like ofc i know you know; just communicating that in a proof can be hard. I am also a bit of a harsh grader tbf)
Are you guys like profs or something?
4th year undergrad
Oh damn
I'm going into a Ph.D. program to be a mathematical logician so slick proofs for things are things I like to come up with. I pretty much leave things in the proof that are sufficient to prove the statement...which I did here too, but making sure the proof flows well and is reader friendly is also good to have other than just correctness.
proofs in textbooks are typically short for a reason
Oh I see, for me I have to add as much detail as possible. Its short but looks complicated lmfao. Also whats a mathematical logician?
Well either way, sounds cool, gl on the phd too
thanks. Proofs should be short and sweet. Some like mine are short and to the point, but may lack some flow that would be easier for readers to digest. A very short description of a mathematical logician is someone who uses tools in logic to go about proving things using tricks in logic that aren't typically taught in a intro to proof class. Proof theory is a huge topic as well.
That sounds difficult as shit
But hey seems like you enjoy what you do so I envy you
haha
Seems super cool though
for instance like this one I posted^ it doesn't seem very obvious how the logic works, but it's there. This is why chatting with other mathematicians is always helpful...especially if I was teaching an undergraduate course. A professor would def understand my proof, but again if you can't convey your proof to ur readers well, it's not really much good if ur audience is not quite grasping what you're putting down
@snow sleet do u happen to know while compiler courses tend to focus on AST and dont use general graphs?
is there no use for general graph theory in those topics
or something along those lines?
i have to take compilers next academic period
and that @jagged grove is the only course I got a C in so I should not answer that question
Graph Theory was taught online and I was left teaching myself it so yea, I know when I shouldn't answer questions and send u down the wrong rabbit hole lol
Fair enough, it would also depend on who you are presenting your proof to. I mean I just started proofs so I am not great at them nor am I decent, I am still learning.
i got a 7 out of 10 in linear algebra cause i didnt take the final exam
i got a 10 in everything though
in graph theory too?
That's why I am supposed to add as much detail as possible ig
@snow sleet i got a b+ in the discrete math
when you're first starting out yea
yo im doing that rn
ah so u didn't cover graph theory as a course itself
One thing graph theory helps us with is some counting problems
and look at more advanced stuff
i know that for sure
good to hear! more exposure the better!
ive looked at a thing called theory of ordered sets
nonempty subset
minimum is a n such that n < a for all a
but yea
<= actually
true
because n<n certainly isn't true
yeah
since n is in the nonempty subset
and were including n as an a
aight yea so u know the WOP then
in that for all quantifier
yup
sec
posets are fun
I am mentoring a directed reading program on them
whats that?
i mean the stuff on that book
pretty sure partially ordered sets are called posets right @faint sphinx ?
correct
so ordered sets would be a subset of posets?
It's a set with an "ordering" on it, which satisfies just 3 axioms/assumptions
i should finish this last problem, no more distraction (i don't know waht im doing)
order i would assume
ordered = poset (or a special type of poset) here. More generally, there are preorders, where you don't assume antisymmetry, but those are less important
Just different naming conventions
seems like the kind of thing that would be important for scheduling systems
me and judson have bad notation in common LMFAOO
I do not know or care for applications 🧍♂️
i gotta make a living
how are u with counting problems?
describe a counting problem quickly
combinatorialists are always in demand
yes
For instance, If S is a set with n elements, how many functions f: S^2 -> S are there?
is s^2 a cartesian product?
yes
SxS=S^2 just like R^2=RxR
way more S^2 is a set of ordered pairs of elements of S
so n^2?
and they are sent to any one of S places right?
nope more
S^2 has n^2 elements
and each of those elements can go to any one of n places
n^3
oh god
im bad?
i know
logician
nope
each element of S^2 ( and there are n^2 of these) goes to any one of n places so n times n times n.... (n^2 times)
(a,b) can go to n places
one of n places
(x,y) can go to one of n places as well
so can all of the ordered pairs
so n^(n^2) is the answer
Now define this for infinite sets 🗿
i dont even know where to start
tbf i don't know like cardinal arithmetic really. But the set of functions A --> B is really just the set B, producted with itself |A| times (lmaoo choice moment for infinite sets)
$\infty^\infty$
logician
LOL
so |A||B| ?
yes
so a can only go to one
yes
for each possible function
infinite sets have cardinals, not size "infinity"
uh
bijections the answer fr
That's the size of A x B
yes\
but we dont want that
cause thats not a function
a function is a subset of AxB
I think that's true? Functions are relations so
yeah but it has more to do with the fact that if (a,b) but if a is in b we cant have (a,a)
I'm asking how many functions we can have....they don't have to be injective
two elements fromt the domain can go to the same element in the co-domain that's totally fine
i edited
yes
cause thats not what i was trying to say
yes
wait are u still trying to count that function question I asked or are u thinking of something else?
im trying to figure out where the n^(n^2) reasoning came from
it came from the fact that the domain has size n^2 and the codomain has size n.
Each element of the domain can be sent to one of n places in the codomain
so u multiply n...n^2 times
question
yes
does this counting behave as a function itself?
what do u mean "this counting"
I mean sure given different n, n^(n^2) should be different right?
Better solution: Consider the set of functions from A --> B, where |A| = N and |B| = M. rather than working around the square in there
well M^A
no
And justify why the | {set of functions A --> B} | = M^N
now this
in Ryx's example the answer should be M^N.
Think about how many elements are in our domain
and how many elements are in our codomain
and use the reasoning that each element of our domain can go to only one element of our codomain
those three things are the only things needed for this problem
the power set is not needed?
you already noted S^2=n^2, and that's correct
dandida is this what you are thinking of
power set is the set of all subsets....yea no we're not doing that here
How about I write S like this instead
sec im going through the justification for m^n
S={s1,s2,...,sn}
Given $S={s_1,s_2,s_3,\dots,s_n}$, count how many functions $f:S^2\rightarrow S$ exist. Recall $S^2={(s_i,s_j):s_i,s_j\in S}$.
functions S --> {0,1} frfr
logician
yeaaaaa buddddddy Ronnie Coleman
do u happend to whatch dota streams or something?
dota streams?
nah I just watched Ronnie deadlift 800lbs and called it "lightweight baby"" ain't nothin to it but to do it
lmao true
I typed this up to help u think about where each element from S^2 goes when it gets sent to S
there is a choosing for each element in the codomain of n
and since we have s^2 elements in the codomain
s can get map s^2 times
nope
in the domain
for each element of the domain [which is of the form (s_i,s_j)] it can be sent to any one of n places. Say it gets sent to s_1. Then for the next ordered pair, it can be sent to one of n places, say s_1 again...and so on
so all the ordered pairs each have n options for where they can go
k
done
uh okay, as long as it makes sense in ur head....what would the answer then be
for what rix mentioned?
no for my problem lol that u were doing
this one^
if u answer it for mine, you'll def be able to answer Ryx's much simpler problem
s^(s*s)
Do mine, and conclude logicians as a special case lol
the reasoning goes both ways. If u generalize ur reasoning
for urs
looks good s=n I'm assuming
yes
perfect
logician
k and n are natural numbers
S^(S^k)
n^(n^k)
now for riz
rizz
can u put the statement back in
so i can put it on paper
and actually write a proof
you could fix one of the sets and induct on the number of elements in the other set
Ryx's or
Yea I'll type it
thats my idea
thx
Given nonempty finite sets $A$ and $B$ with $|A|=N$ and $|B|=M$, count all the possible functions $f:A\rightarrow B$.
ur missing the second part
logician
u mean the answer?
this
i meant to prove that by induction
Given nonempty finite sets $A$ and $B$ with $|A|=N$ and $|B|=M$, count all the possible functions $f:A\rightarrow B$. Justify why this answer is $M^N$.
logician
ooh no not induction
why?
shhhh
That doesn't really fit with this one
jk
it does if u fix the correct set. My prof did this once
Yeah but you don't need it anyways, and I don't think it helps here
its ok
writing give me a sec
Think about which set ur going to fix the cardinality of
Ping me when u have a proof I’ll be grabbing some food in the meantime but yea I wanna see this. I recommend induction on |the cardinality of A.|
Shoot was trying to make that one of those black things that u can open up to see
This looks fine for n=1, but the ordered pairs look backwards and the last line “which means…” isn’t convincing me that the number of functions when n is 1 is precisely m
thats what was bugging me
how can i express the counting of the possible functions
like i have shown that for a function there are m elements sure
its what i figured
If A={a}, (a, b_1) etc
U do it how u want, some people do with words or just use ordered pairs like I did
(a,b_1) represents one function
one thing though is this still discrete math?
(a, b_2) another etc
Sure is
Discrete math is the overarching area combinatorics is the more precise area
ur right my pairs are backwards
Im glad u fixed the right set tho ahaha
K
did she mean m + 2 = 2?
looks good to me
nope this proof is right
oh n is also 0 I see
i was thinking more like n+2m+1>= n+2*1+1
n being a perfect square means its nonnegative
since m>=1
technically ur right
that should be a >= instead of > there
u go spreading that formalism on everyone jo
jeez
lol
let me go back to the other proof
lmao
why did she replace m with 1 in n + 2 * 1 + 1
to show it's at least that number
on her way to showing it's greater than n+2
because m is at least 1
oh ok I see
again it shouldn't be a strict inequality between n+2m+1 and n+2*1+1, but nevertheless ur teacher prolly already knows about that typo
since m could be 1
hopefully this gets easier with practice
it will
different ppl prove things differently
everyone has their own flavor
sometimes it's hard reading other proofs
thats why reading different books makes things easier
I'm typing up a proof by induction for our problem rn
(this formula still makes sense when A or B is empty btw)
done
damn
give me a sec to show u how i was approaching it
ill finish the step where im at
i was thinking the exact same thing but was expressing it in a cumbersome way
let me show u
aight
why are we supposing n^3 + 5 is odd?
since its contradiction aren't we supposed to say n3+ 5 is even and n is odd
U could define T more concisely as $T={(a_i,b_j):i\in{1,2,3,\dots,n},\ j\in{1,2,3,\dots,m}}$
logician
nope proof by contradiction only assumes the negation of the consequent not the negation of the atecedent
we can assume n is an integer and that n^3+5 is odd...prove by contradiction means we also add the assumption n is not even (i.e., n is odd in this case)
but you proof that a then not b leads to a contradiction
then the case true then false disappears
oh ok I'll write that down
so ur left with truth
this is the thing though, sometimes i have the right idea but idk how to express it in a way in which im minimizing the amount of effort it takes to write it
let me give u an example of a proof by contradiction
just takes practice
do the inexistence of the square root of 2 in the rationals
done
are u a phd student by any chance?
almost
at masters?
nah I'm a senior undergrad rn
But let me write a proof that more fits the structure of the statement you showed.
doing it with no suprema and axiom of completeness
that is new
nvm
ur proving that is irrational
not that it exists
Given $x\in\mathbb R$, if $0\leq x<\epsilon$ for each $\epsilon>0$, then $x=0$
logician
@plucky wedge I'll write a proof by contradiction for this problem
they dont see neighboorhoods in discrete how u going to show that?
\begin{proof}Suppose $x\neq0$. Then $x>0$, and so $0<x<x$, a contradiction. \end{proof}
nvm
logician
divides
oh ok
q^2=p^2/2 from the equation 2q^2=p^2
oh
Are u looking at the first or second proof for the sqrt of 2 being irrational?
they're both by contradiction
but yea which one are u looking at
first one
k
the math makes sense now but what is gcd(p, q)
greatest common divisor
greatest common factor
nope we're given x>=0
ur not wrong to say greatest common divisor
GCD literally stands for that
it means p and q share no other positive divisor other than 1
gcd(p,q)=1
but I showed they share 2 as well
a contradiction
in analysis sherbert uses what u showed alot for reaching contradictions
things like the limit of a convergent sequence is unique
and stuff of that nature
but he shows it using neighborhoods
idk why
like epsilon neighboorhoods
the definition of a sequence converging literally has epsilon neighborhoods in the definition of convergence
the definition says
so it shows 2 is a common factor of p and q but why do we say that p and q share no other positive divisor other than 1?
is it implied in the proof somewhere
primes
or a definition of something
let $(a_n)_{n=1}^\infty$ be a sequence in $\mathbb R$. Then we say the sequence converges to the point $L\in\mathbb R$ if for every $\epsilon>0$, there is a natural number $N$ for which $|a_n-L|<\epsilon$ for all $n\geq N$. In this case we say $a_n\rightarrow L$
logician
you can assume that all common factors of p and q have been taken out aside from 1
in other words, u can assume the fraction is a simplified as possible
wouldnt u have to add something like
that's an assumption u can make without losing cases in ur proof
withouth loss of generality
ok
u could and u wouldn't be wrong
but it's not needed. If I give u a fraction u can assume it's in it's most simplified form, otherwise simplify it until u get gcd(p,q)=1
oh ok so we are assuming that p/q is in it's most simplified form but since we can take out 2 it's a contradiction
yes
yes
I assumed p and q were relatively prime (meaning they share no other positive factor other 1), and yet I showed p and q are both even and hence divisible by 2>1
yup that's it
@jagged grove do u see why epsilon is needed now lol
oh ok I see
my second proof for that makes use of the fact that I can assume p is minimal
given a nonempty set of positive integers, the well-ordering principle allows me to make such an assumption
well cause we need any epsilon greater than 0
by def of convergence
we should probably tell ryx that we showed the problem suggested
yes and $|a_n-L|<\epsilon$ is equivalent to saying $L-\epsilon<a_n<L+\epsilon$
logician
is proofs usually the first unit in discrete math
that can be shown using the properties of the absolute value
depends on whos giving it
and the fact that epsilon was positive
not usually. usually it's set theory
and then logic
and then proofs potentially
and some colleges dont even emphasize proofs
like they mostly give u induction
and call it a day
in their defense
at least thats what they did in my college for SE majors
u can't bring someone from intro level to wizard level in even a semester

proof theory takes time to learn
true
lol
and even profs aren't quite at the finish line of wizardly level. There merely much further along the math proof experience level than students are
no clue
at UCR where i did math (i can actually go back whenever i want i just dont have the money to. If i get a permanent position that might be different though).
every teacher needs a phd
to give a undergrad level course
its mandatory
idk how common that is
well yea because profs are representing the college and ppl pay a lot of money to go to universities/colleges. They want their profs to be experts in their fields
in UCR its barely paying i could take an entire undergrad semester for about 300 dollars
so
this is american mind u
and i had no scholarship
let me send u a link
k
costa rica
I mean think about it this way: If you're a prospective student looking to apply for colleges and one college has professors all with Ph.D.'s while another has only some with Ph.D.'s, that could be a deciding factor....so not to surprising to see colleges wanting profs with Ph.D's
yeah but look at the colleges though
of course
the ones from latin america are top latin american unis
and most of them are from france and the us
but yea aside from campus and such, I'd wanna be taught by an expert especially since I'm paying a lot for education
where are u at the moment?
I got a proof for u to try @jagged grove
sure
west coast united states
Show that there are infinitely many even numbers greater than two that can be written as the sum of two primes
prove this directly
isnt this a variation of the fundamental theorem of arithmetic?
not really. I made up this problem
that has to do with a unique product of primes.
Ryx let him think
two cannot even be written as the sum of two primes
unneeded condition
it was also my way of stating positive evens
fair
I'm curious to see what dandida comes up with
U can assume there are infinitely many primes
so this is different from goldbachs cause of the infinitely many?
more or less yes
goldbach is that every even > 2 can be writen like that
this just just infinitely many
basically
My hints for u @jagged grove are u can prove this directly and u may assume there are infinitely many primes
so would showing that there exists more than 1 be enough for this?
this is what i dont get though
i dont recall being introduced to the infinitely many
concept
okay,
so
if I asked u to prove there are infinitely many odd numbers
u could state, let z be an integer then 2z+1. Moreover, 2z+1<2(z+1)+1.
There are infintely many integers
and I showed there are infintely many odd numbers I can construct
does that make sense?
okay use a similar method of "construction" for when u show that there are infinitely many even numbers greater than two that can be written as the sum of two primes
yes there are infintely many distinct integers, and I showed that that corresponds to distinct 2z+1. In other words if u gave me distinct z and y, then 2z+1 would not equal 2y+1.
So we haven't lost the infinitism so to speak