Hi,

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.

https://www.mapleprimes.com/questions/37228-Ignore-Terms-Above-Particular-Orderpower

(Error, selecting function must return true or false)

Thanks a lot,

Rahul

I am defining, 

f(x):= x-> diff(g(x),x)/(1+g(x))

Here g(x)<<1 and I am looking for series expansion, 1/(1+g(x)) = 1- g(x) + g(x)^2 etc.

But I cannot get this by using series(f(x),g(x)=0) or other options.

Thanks,

Rahul