Compositions and parities of complete mappings and of orthomorphisms

TL;DR

This paper characterizes the permutation groups generated by complete mappings and orthomorphisms of finite groups and fields, showing they are typically the full symmetric group except for small cases, with implications in cryptography and Latin squares.

Contribution

It determines the permutation groups generated by complete mappings and orthomorphisms for finite fields and groups, extending known results and analyzing their parities.

Findings

For finite fields, the groups are the full symmetric group except for small cases.

For large enough finite groups with complete mappings, the generated groups are the full symmetric or at least the alternating group.

The study of parities of these mappings connects to cryptography and Latin square parity types.

Abstract

We determine the permutation groups $P_{\mathrm{comp}}(\mathbb{F}_q),P_{\mathrm{orth}}(\mathbb{F}_q)\leq\operatorname{Sym}(\mathbb{F}_q)$ generated by the complete mappings, respectively the orthomorphisms, of the finite field $\mathbb{F}_q$ -- both are equal to $\operatorname{Sym}(\mathbb{F}_q)$ unless $q\in\{2,3,4,5,8\}$. More generally, denote by $P_{\mathrm{comp}}(G)$, respectively $P_{\mathrm{orth}}(G)$, the subgroup of $\operatorname{Sym}(G)$ generated by the complete mappings, respectively the orthomorphisms, of the group $G$. Using recent results of Eberhard-Manners-Mrazovi\'c and M\"uyesser-Pokrovskiy, we show that for each large enough finite group $G$ that has a complete mapping (i.e., whose Sylow $2$-subgroups are trivial or noncyclic), $P_{\mathrm{comp}}(G)=\operatorname{Sym}(G)$ and $P_{\mathrm{orth}}(G)\geq\operatorname{Alt}(G)$. We also prove that $P_{\mathrm{orth}}(G)=\operatorname{Sym}(G)$ for every large enough finite solvable group $G$ that has a complete mapping. Proving these results requires us to study the parities of complete mappings and of orthomorphisms. Some connections with known results in cryptography and with parity types of Latin squares are also discussed.