what is the "more general object" about Gram matrix?

[felixdae]

in you wonderful blog "Convolutional neural networks for artistic style transfer", you said "the Gram matrix is a special case of a more general object". what is the general object, can you please give some reference? thank you.

[hnarayanan]

This is a deep question. I will answer it when I get some time. :)

[felixdae]

is there some paper or book about it? maybe i can try to understand it at first. :)