I found this question generalization of geometric series, which asks about generalization of geometric series. Unfortunately, there is no answer and seems there is no such things.
Anyhow, in my case I don't have a polynomial exponent, but a specific case with square root: $$\sum_{n=0}^\infty a^{\sqrt{n}},\ |a| < 1,$$
and in my case $a = e^{-t}$. Is there maybe some closed-form solution in terms of e.g. theta-function, or this is even more lost case than a polynomial one? Thanks.