Involutary pemutations over finite fields given by trinomials and quadrinomials

Abstract

For all finite fields of $q$ elements where $q\equiv1\pmod4$ we have constructed permutation polynomials which have order 2 as permutations, and have 3 terms, or 4 terms as polynomials. Explicit formulas for their coefficients are given in terms of the primitive elements of the field. We also give polynomials providing involutions with larger number of terms but coefficients will be conveniently only two possible values. Our procedure gives at least $(q-1)/4$ trinomials, and $(q-1)/2$ quadrinomials, all yielding involutions with unique fixed points over a field of order $q$. Equal number of involutions with exactly $(q+1)/2$ fixed-points are provided as quadrinomials.