A summation that "feeds back into itself"

[tovisonnenberg]

Hi everyone! I need to use sigma notation to build a summation that "feeds back into itself". By that I mean that it should model a sum whose terms are f(x) + f(f(x)) + f(f(f(x))) and so on. How would I do this?

 

[mfb]

You can write f(f(f(x))) as f³(x) and that 3 can be replaced by a variable, too.