# Bilinear response

1. Feb 11, 2015

### hokhani

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

2. Feb 11, 2015

### DrDu

Context?

3. Feb 11, 2015

### hokhani

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

4. Feb 11, 2015

### DrDu

"In a paper". Do you think this is helpful? Maybe you could give a citation and quote the relevant part?

5. Feb 11, 2015

### hokhani

6. Feb 12, 2015

### 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 $\phi^*$.