Operation algebra: prove [A,f(A)]=0

[itai]

Homework Statement

given an opeator A, prove that the commutator satisfies:
[A,f(A)]=0

Homework Equations

[A,f(A)]=Af(A)-f(A)A

The Attempt at a Solution

I don't know how to continue from Af(A)-f(A)A=0
I can't just say that f(A)A=Af(A).
Any ideas?

 

[quZz]

if f(z) is analytical then you have f(z) = f0 + f1 z + f2 z² + ...