#Find an affinity

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nimble island
#

Hello, I am studying affine geometry, particularly the theorem attached as an image. I asked ChatGPT to give me a numerical example of this (since the book doesn't include one). I would like to know if it is correct. It seems strange to me that it doesn't consider the vector (\overrightarrow{X_2X_3}) and (\overrightarrow{Y_2Y_3}). Also, in step 2, I don't understand how it was converted into matrix form. :c

Given the vertices:

\begin{itemize}
\item (X_1(1,1)), (X_2(2,3)), (X_3(4,2))
\item (Y_1(2,2)), (Y_2(3,5)), (Y_3(6,4))
\end{itemize}

\subsubsection{Step 1: Calculate the vectors of the triangles}

For the triangle (X_1X_2X_3):

[
\overrightarrow{X_1X_2} = \begin{pmatrix} 2 - 1 \ 3 - 1 \end{pmatrix} = \begin{pmatrix} 1 \ 2 \end{pmatrix}, \quad \overrightarrow{X_1X_3} = \begin{pmatrix} 4 - 1 \ 2 - 1 \end{pmatrix} = \begin{pmatrix} 3 \ 1 \end{pmatrix}
]

For the triangle (Y_1Y_2Y_3):

[
\overrightarrow{Y_1Y_2} = \begin{pmatrix} 3 - 2 \ 5 - 2 \end{pmatrix} = \begin{pmatrix} 1 \ 3 \end{pmatrix}, \quad \overrightarrow{Y_1Y_3} = \begin{pmatrix} 6 - 2 \ 4 - 2 \end{pmatrix} = \begin{pmatrix} 4 \ 2 \end{pmatrix}
]

\subsubsection{Step 2: Set up the system for the matrix (L)}

We seek the matrix (L) such that:

[
L \begin{pmatrix} 1 \ 2 \end{pmatrix} = \begin{pmatrix} 1 \ 3 \end{pmatrix}, \quad L \begin{pmatrix} 3 \ 1 \end{pmatrix} = \begin{pmatrix} 4 \ 2 \end{pmatrix}
]

This can be written in matrix form as:

[
L \begin{pmatrix} 1 & 3 \ 2 & 1 \end{pmatrix} = \begin{pmatrix} 1 & 4 \ 3 & 2 \end{pmatrix}
]

south parrotBOT
#
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sacred cosmosBOT
#

Mαя

LaTeX source sent via direct message.
```Compilation error:```! Missing $ inserted.
<inserted text> 
                $
l.49 ...nsider the vector (\overrightarrow{X_2X_3}
                                                  ) and (\overrightarrow{Y_2...
I've inserted a begin-math/end-math symbol since I think
you left one out. Proceed, with fingers crossed.

LaTeX Font Info:    Trying to load font information for OT1+lmr on input line 4
9.
(/usr/local/texlive/2023/texmf-dist/tex/latex/lm/ot1lmr.fd```
shell raven
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it's better if you formulate your question, I am not gonna read whatever gpt generated :S

#

do you want a proof of the theorem? specific example of an affinity?

nimble island
#

And specific example to understand what the theorem is saying to me

shell raven
#

let's start from scratch then

nimble island
#

Mm we have six points, X_1, ... and Y_1, ... and we have to find a function that send each X_i point to Y_i where i = 1,2,3

#

We know that the function is an affinity and have the form of L*x + b, where L is the transformation matrix, and b the traslation vector

shell raven
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Im assuming these are proper triangles

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if it already tells you that X_i is mapped to Y_i

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then that defines the map

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verify that it results in an affinity

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and given any affinity that maps X_i to Y_i, it must coincide with the one you started with

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@nimble island

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but if you know how vector spaces work, then there's really nothing to prove here

wary slate
nimble island
wary slate
#

I mean, we can set up a system of the form AX + B = X', but that results in 6 equations in, I believe, 10 unknowns.

shell raven
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but the proof is constructive no?

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can't you simply substitute specific data

nimble island
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Mm I don't understand why is enought with only use X_1X_2 and X_1X_3? why X_2X_3 doesn't matter? I need three lines to have a triangle

nimble island
shell raven
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any proper triangle is an affine basis

#

see the theorem they mention here

wary slate
nimble island
# shell raven any proper triangle is an affine basis

my lack on english and understanding of the subject is giving problems to understand what are you trying to say to me, what is an afifne basis? is the basis vector that have a point P who belongs to an affine space?

wary slate
nimble island
wary slate
#

It's a combination of a linear transformation and a translation.

wary slate
nimble island
#

Siuuuuu