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0 pointssetopt4mo ago0 comments

> > Just intuitively, in such a high dimensional space, two random vectors are basically orthogonal.

> What's the intuition here? Law of large numbers?

Imagine for simplicity that we consider only vectors pointing parallel/antiparallel to coordinate axes.

- In 1D, you have two possibilities: {+e_x, -e_x}. So if you pick two random vectors from this set, the probability of getting something orthogonal is 0.

- In 2D, you have four possibilities: {±e_x, ±e_y}. If we pick one random vector and get e.g. +e_x, then picking another one randomly from the set has a 50% chance of getting something orthogonal (±e_y are 2/4 possibilities). Same for other choices of the first vector.

- In 3D, you have six possibilities: {±e_x, ±e_y, ±e_z}. Repeat the same experiment, and you'll find a 66.7% chance of getting something orthogonal.

- In the limit of ND, you can see that the chance of getting something orthogonal is 1 - 1/N, which tends to 100% as N becomes large.

Now, this discretization is a simplification of course, but I think it gets the intuition right.

0 comments

nyeah4mo ago

I think that's a good answer for practical purposes.

Theoretically, I can claim that N random vectors of zero-mean real numbers (say standard deviation of 1 per element) will "with probability 1" span an N-dimensional space. I can even grind on, subtracting the parallel parts of each vector pair, until I have N orthogonal vectors. ("Gram-Schmidt" from high school.) I believe I can "prove" that.

So then mapping using those vectors is "invertible." Nyeah. But back in numerical reality, I think the resulting inverse will become practically useless as N gets large.

That's without the nonlinear elements. Which are designed to make the system non-invertible. It's not shocking if someone proves mathematically that this doesn't quite technically work. I think it would only be interesting if they can find numerically useful inverses for an LLM that has interesting behavior.

All -- I haven't thought very clearly about this. If I've screwed something up, please correct me gently but firmly. Thanks.

zipy1244mo ago

for 768 dimensions, you'd still expect to hit (1-1/N) with a few billion samples though. Like that's a 1/N of 0.13%, which quite frankly isn't that rare at all?

Of course are vectors are not only points in one coordinate axes, but it still isn't that small compared to billions of samples.

sigmoid104mo ago

Bear in mind that these are not base vectors at this stage (which would indeed give you 1/768). They are arbitrary linear combinations. There are exponentially many near orthogonal of these vectors for small epsilon. And epsilon is chosen pretty small in the paper.

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