On the $q$-analogue of P\'olya's Theorem.

We answer a question posed by Michael Aissen in 1979 about the $q$-analogue of a classical theorem of George Polya (1922) on the algebraicity of (generalized) diagonals of bivariate rational power series. In particular, we prove that the answer to Aissen's question, in which he considers $q$ as a variable, is negative in general. Moreover, we show that the answer is positive if and only if $q$ is a root of unity.