How can I perform a summation of a freqz function

Hi, I'm currerntly trying to figure out how to perform a summation of a frequency transfer function. How would I go about summizing a freqz function of z^-k for any given values. For example, if I wanted to range k from 0-3, how would I arrange the sumsym function to evaluate the following transfer function of 1+z^-1+z^-2+z^-3. Thanks for any help.

Answer 1

[Torsten]

syms z k 
F = @(a) symsum(z^(-k),k,0,a);
F(2)

ans =