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> they do compute a "correlation coefficient" between two people by adding together the products of scores each gave to a post

I've heard the term "cosine similarity" before but not really looked into it. What does this computation have to do with trigonometry?

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The dot product is computed between two vectors. For these use cases that dot product is equal to the cosine of the angle between these angles.

(Strictly speaking we have that the angle is actually defined in terms of the dot/inner product in more abstract spaces like function spaces or L^p/l^p)

It's grounded in basic trigonometry, i.e. it calculates the angle `theta` between two entities/vectors, `a` and `b`. If `theta` is close to 180 degrees, cos(theta) is -1, and cosine similarity dictates these are opposite concepts, i.e. unrelated.

Unrelated would be a cosine of 0. Being opposites is a very strong relation.

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