Indecomposable rational functions over finite fields with Galois closure of genus one: Reconstruction and exceptionality

Abstract

Let $k=\mathbb F_q$. We reconstruct every $k$-indecomposable $g\in k(X)$ of degree greater than one whose normal closure has genus one. Such a map is automatically separable and, after independent degree-one changes of the source and target coordinates over $k$, arises from a separable equivariant isogeny between elliptic curves equipped with compatible finite group actions stable under Frobenius conjugation. In particular, $\deg g=\ell$ or $\ell^2$ for a prime $\ell$, with $\ell\ne\operatorname{char}k$ in the latter case. More generally, every separable rational function with genus-one Galois closure admits a canonical factorization class, modulo degree-one changes of the intermediate coordinates over $k$, determined by the intrinsic translation subgroup of its geometric monodromy group; every $k$-indecomposable factor of the remaining map has Galois closure of genus zero. For every equivariant-isogeny quotient and every finite extension $k_r/k$, the same exact finite-kernel condition characterizes both permutation of $\mathbf P^1(k_r)$ and exceptionality over $k_r$. The finite kernel and its induced Frobenius and linear symmetry actions also determine the arithmetic and geometric monodromy permutation groups, decomposition classes, and the exact periodic set of permutation extension degrees, including its least period and limiting proportion. Together with the corresponding genus-zero theorem, these results yield permutation if and only if exceptionality over every finite extension for every separable rational function whose Galois closure has genus at most one; under $k$-decomposition, the common extension-degree set is the intersection of the corresponding sets for the factors.