An Artin-Schreier polynomial over a field F is of the form
for α in F. The function is p-to-one since . It is in fact -linear on F as a vector space, with kernel the one-dimensional subspace generated by , that is, itself.
Suppose that F is finite of characteristic p. The Frobenius map is an automorphism of F and so its inverse, the p-th root map is defined everywhere, and p-th roots do not generate any non-trivial extensions.
If F is finite, then A is exactly p-to-1 and the image of A is a -subspace of codimension 1. There is always some element α of F not in the image of A, and so the corresponding Artin-Schreier polynomial has no root in F: it is an irreducible polynomial and the quotient ring is a field which is a degree p extension of F. Since finite fields of the same order are unique up to isomorphism, we may say that this is "the" degree p extension of F. As before, both roots of the equation lie in the extension, which is thus a splitting field for the equation and hence a Galois extension: in this case the roots are of the form .