On the group of unit-valued polynomial functions

Let R be a finite commutative ring. The set F(R) of polynomial functions on R is a finite commutative ring with pointwise operations. Its group of units F(R) ^× is just the set of all unit-valued polynomial functions. We investigate polynomial permutations on R[x] / (x ²) = R[α] , the ring of dual numbers over R, and show that the group P _R(R[α]) , consisting of those polynomial permutations of R[α] represented by polynomials in R[x], is embedded in a semidirect product of F(R) ^× by the group P(R) of polynomial permutations on R. In particular, when R= F _q, we prove that PFq(Fq[α])≅P(Fq)⋉θF(Fq)×. Furthermore, we count unit-valued polynomial functions on the ring of integers modulo p ⁿ and obtain canonical representations for these functions.