# Bilinear response

## Main Question or Discussion Point

I don't know what is "bilinear responce". I would be grateful for any help.

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DrDu
Context?

Context?
Yes, I saw the term "bilinear response" in a paper and I don't understand it.

DrDu
It means that it is linear in both $\delta X_1$ and $\delta X_2$. This is generally used for expressions of the form xy. Also the scalar product $\langle \phi|\psi \rangle$ is bilinear in $\psi$ and $\phi^*$.