@Mariusz Iwaniuk 

Thank you for your response. I have now the problem with that expression, that for certain values the expression is complex with an imaginary part of 0.*I or 10^-30 and so on. Here is an code example.

INV := invztrans((z - 1)^2/(a*z^2 + b*z + c), z, n);
INVnew := simplify(allvalues(INV));
evalf(subs(a = 2, b = 1, c = 4, eval(subs(n = 16, INVnew))));

#-125.345817565917968749999999999999999999999999999999999999998 - 0.*I

The problem is that I am using an optimization methode after this, and the Re() command out of Maple takes a lot of time to evaluate if a,  b, and c are not substituted. Do you have any idea for that?