Let $f(T) = \sum a\_n T^n \in \mathbf{F}\_p [[ T ]]$ be a power series. We'll say that the coefficients of $f(T)$ are equidistributed modulo $p$ if for every residue class $a$ modulo $p$, we have
$$ \lim\_{X \to \infty} \dfrac{1}{X} \cdot \# \{n<X : a\_n \equiv a \mod p \} = \dfrac{1}{p}.$$  

Suppose that $f(T), g(T) \in \mathbf{F}\_p [[ T ]]$ are power series whose coefficients are equidistributed modulo $p$ in the above sense. Is it true that the coefficients of the product $f(T) \cdot g(T)$ are also equidistributed modulo $p$?



Is the product of two equidistributed power series equidistributed?
No. Let $f(T)$ be chosen uniformly at random from all power series with constant term nonzero and let $g(T) = 1/ f(T) $. Then $g(T)$ is also chosen uniformly at random from all power series with constant term nonzero. (Here uniformly means each possible sequence of first $n$ coefficients has equal probability, and the proof is just that the first $n$ coefficients of $f$ determine the first $n$ coefficients of $g$ and vice versa.)


So $f(T)$ and $g(T)$ are each equidistributed with probability $1$ but $f(T)g(T)=1$ is not equidistributed.


The same argument shows that any condition on $f$ and $g$ that is "generic" in the sense of holding for a random power series is not sufficient.


