I read the following statement in an article:
Suppose $(a_k) $ is a nonnegative sequence and let $a(x)=\sum_{k=0}^{\infty}a_kx^k$. If $a(x)$ is not a power series in $x^t$ for any integer $t>1$, then....
But, I am confused with "not a power series". Does anyone please give me an example of this? Thank you very much for your help.