Permutation polynomials of finite fields

Christopher J. Shallue

Published 2012-11-23Version 1

Let $\mathbb{F}_q$ be the finite field of $q$ elements. Then a \emph{permutation polynomial} (PP) of $\mathbb{F}_q$ is a polynomial $f \in \mathbb{F}_q[x]$ such that the associated function $c \mapsto f(c)$ is a permutation of the elements of $\mathbb{F}_q$. In 1897 Dickson gave what he claimed to be a complete list of PPs of degree at most 6, however there have been suggestions recently that this classification might be incomplete. Unfortunately, Dickson's claim of a full characterisation is not easily verified because his published proof is difficult to follow. This is mainly due to antiquated terminology. In this project we present a full reconstruction of the classification of degree 6 PPs, which combined with a recent paper by Li \emph{et al.} finally puts to rest the characterisation problem of PPs of degree up to 6. In addition, we give a survey of the major results on PPs since Dickson's 1897 paper. Particular emphasis is placed on the proof of the so-called \emph{Carlitz Conjecture}, which states that if $q$ is odd and `large' and $n$ is even then there are no PPs of degree $n$. This important result was resolved in the affirmative by research spanning three decades. A generalisation of Carlitz's conjecture due to Mullen proposes that if $q$ is odd and `large' and $n$ is even then no polynomial of degree $n$ is `close' to being a PP. This has remained an unresolved problem in published literature. We provide a counterexample to Mullen's conjecture, and also point out how recent results imply a more general version of this statement (provided one increases what is meant by $q$ being `large').