Permutation polynomials from a linearized decomposition

Abstract

In this paper we discuss the permutational property of polynomials of the form $f(L(x))+k(L(x))\cdot M(x)\in \mathbb F_{q^n}[x]$ over the finite field $\mathbb F_{q^n}$, where $L, M\in \mathbb F_q[x]$ are $q$-linearized polynomials. The restriction $L, M\in \mathbb F_q[x]$ implies a nice correspondence between the pair $(L, M)$ and the pair $(g, h)$ of conventional $q$-associates over $\mathbb F_q$ of degree at most $n-1$. In particular, by using the AGW criterion, permutational properties of our class of polynomials translates to some arithmetic properties of polynomials over $\mathbb F_q$, like coprimality. This relates the problem of constructing PPs of $\mathbb F_{q^n}$ to the problem of factorizing $x^n-1$ in $\mathbb F_q[x]$. We then specialize to the case where $L(x)$ is the trace polynomial from $\mathbb F_{q^n}$ over $\mathbb F_q$, providing results on the construction of permutation and complete permutation polynomials, and their inverses. We further demonstrate that the latter can be extended to more general linearized polynomials of degree $q^{n-1}$.