Gram matrix
Consider -vectors . The Gram matrix of the collection is the matrix with elements . The matrix can be expressed compactly in terms of the matrix , as
By construction, a Gram matrix is always symmetric, meaning that for every pair . It is also positive semi-definite, meaning that for every vector (this comes from the identity ).
Assume that each vector is normalized: . Then the coefficient can be expressed as
where is the angle between the vectors and . Thus is a measure of how similar and are.
The matrix arises for example in text document classification, with a measure of similarity between the th and th document, and their respective bag-of-words representation (normalized to have Euclidean norm 1).
See also: