Low-degree permutation rational functions over finite fields
Volume 202 / 2022
Abstract
We determine all degree- rational functions f(X)\in \mathbb {F}_q(X) which permute \mathbb {P}^1(\mathbb {F}_q), and answer two questions of Ferraguti and Micheli about the number of such functions and the number of equivalence classes of such functions up to composing with degree-one rational functions. We also determine all degree-8 rational functions f(X)\in \mathbb {F}_q(X) which permute \mathbb {P}^1(\mathbb {F}_q) in case q is sufficiently large, and do the same for degree 32 in case either q is odd or f(X) is a nonsquare. Further, for thousands of other positive integers n, for each sufficiently large q we determine all degree-n rational functions f(X)\in \mathbb {F}_q(X) which permute \mathbb {P}^1(\mathbb {F}_q) but which are not compositions of lower-degree rational functions in \mathbb {F}_q(X). Some of these results are proved by using a new Galois-theoretic characterization of additive (linearized) polynomials among all rational functions, which is of independent interest.