Functional degrees and arithmetic applications II: The Group-Theoretic Prime Ax-Katz Theorem

We give a version of Ax-Katz's $p$-adic congruences and Moreno-Moreno's $p$-weight refinement that holds over any finite commutative ring of prime characteristic. We deduce this from a purely group-theoretic result that gives a lower bound on the $p$-adic divisibility of the number of simultaneous zeros of a system of maps $f_j: A\to B_j$ from a fixed ``source'' finite commutative group $A$ of exponent $p$ to varying ``target'' finite commutative $p$-groups $B_j$. Our proof combines Wilson's proof of Ax-Katz over $\mathbb{F}_p$ with the functional calculus of Aichinger-Moosbauer.