Existence results for cyclotomic orthomorphisms

Abstract

An {\em orthomorphism} over a finite field $\mathbb{F}$ is a permutation $\theta:\mathbb{F}\mapsto\mathbb{F}$ such that the map $x\mapsto\theta(x)-x$ is also a permutation of $\mathbb{F}$. The orthomorphism $\theta$ is {\em cyclotomic of index $k$} if $\theta(0)=0$ and $\theta(x)/x$ is constant on the cosets of a subgroup of index $k$ in the multiplicative group $\mathbb{F}^*$. We say that $\theta$ has {\em least index} $k$ if it is cyclotomic of index $k$ and not of any smaller index. We answer an open problem due to Evans by establishing for which pairs $(q,k)$ there exists an orthomorphism over $\mathbb{F}_q$ that is cyclotomic of least index $k$. Two orthomorphisms over $\mathbb{F}_q$ are orthogonal if their difference is a permutation of $\mathbb{F}_q$. For any list $[b_1,\dots,b_n]$ of indices we show that if $q$ is large enough then $\mathbb{F}_q$ has pairwise orthogonal orthomorphisms of least indices $b_1,\dots,b_n$. This provides a partial answer to another open problem due to Evans. For some pairs of small indices we establish exactly which fields have orthogonal orthomorphisms of those indices. We also find the number of linear orthomorphisms that are orthogonal to certain cyclotomic orthomorphisms of higher index.