# Cross term gives the autocorrelation?

1. Aug 22, 2008

### SamTaylor

at the beginning there is an expression I don't unterstand

http://www.wellesley.edu/Physics/brown/pubs/acv85_P1595-P1601.pdf

Does someone know what is meant with the word cross term?

2. Aug 22, 2008

### mathman

When you expand (a+b)2, it becomes a2 +2ab +b2.

The term 2ab is the cross term. What you want is gotten by expanding the integrand to get 3 integrals and using the middle one, omitting the 2.

3. Aug 23, 2008

Thanks