this post was submitted on 15 Mar 2025
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Science Memes

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[–] neidu3@sh.itjust.works 37 points 1 day ago* (last edited 1 day ago) (6 children)

Could one of you scholars please explain the joke for us smoothbrains who don't get it? All I see is a boolean matrix, and I'm not even sure that is correct.

[–] zqwzzle@lemmy.ca 68 points 1 day ago

A square matrix with the ones in the diagonal is called the identity matrix

[–] lugal@lemmy.dbzer0.com 40 points 1 day ago

It's an identity matrix. You multiple a vector with it and the result is still the same (identical) vector

[–] pseudo@jlai.lu 13 points 1 day ago* (last edited 1 day ago) (1 children)

That matrix of zeros with one in diagonal is called the matrix of identity.
It is famous because when doing multiplication on matrix or vector, it acts likes 1 on "normal" number:
x times 1 is x anyMatrix times Identity is anyMatrix.

[–] fallingcats@discuss.tchncs.de 4 points 1 day ago (2 children)

Wouldn't you need to put anyMatrix first, since matrix multiplication isn't commutative?

[–] pseudo@jlai.lu 2 points 1 day ago (1 children)

You are right. I will correct it.

[–] fallingcats@discuss.tchncs.de 1 points 1 day ago* (last edited 17 hours ago) (1 children)

No I just tried it and I was wrong, it seems like it doesn't matter for the ID matrix specifically

[–] pseudo@jlai.lu 1 points 20 hours ago

I need to freshen up on my linear algebra. I don't remember on which sense it goes but I think if you swap the factors, you'll have to you transpose the matrix x to keep the same result.

[–] MBM@lemmings.world 1 points 1 day ago

Huh? Identity times anyMatrix is also anyMatrix. The matrix just has to have the right dimensions

[–] Gustephan@lemmy.world 13 points 1 day ago

Eigen see how this is confusing, I don't get it either

[–] Saleh@feddit.org 5 points 1 day ago

You multiply vectors and matrizes row by column.

So for any matrix the fitting identity matrix multiplies each row on the relevant position by one and puts it into a column.

The matrix remains the same.

See example 5 here: https://www.geeksforgeeks.org/identity-matrix/

[–] ryedaft@sh.itjust.works 2 points 1 day ago

It's like if you multiply or divide by one. Just a bit bigger cuz linear algebra.