Question: ignoring coefficients of higher order terms


I am collecting first order terms in psi and phi in the following expression.

Basically I want to keep coefficients for which (power of phi + power of psi) < 2 and ignore the rest.

Here, H, a, psi, phi are all functions, not variabels.

I tried following answer but it doesn't work due to derivatives.

(Error, selecting function must return true or false)

Thanks a lot,


Please Wait...