this post was submitted on 15 Mar 2025
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Science Memes

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identity theft (lemmy.dbzer0.com)
submitted 3 days ago by [email protected] to c/[email protected]
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[–] [email protected] 37 points 3 days ago* (last edited 3 days 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.

[–] [email protected] 14 points 2 days ago* (last edited 2 days 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.

[–] [email protected] 5 points 2 days ago (2 children)

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

[–] [email protected] 1 points 2 days ago

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

[–] [email protected] 3 points 2 days ago (1 children)

You are right. I will correct it.

[–] [email protected] 1 points 2 days ago* (last edited 2 days ago) (1 children)

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

[–] [email protected] 1 points 2 days 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.

[–] [email protected] 5 points 3 days 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/

[–] [email protected] 16 points 3 days ago

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

[–] [email protected] 2 points 3 days ago

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

[–] [email protected] 41 points 3 days ago

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

[–] [email protected] 70 points 3 days ago

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