Newton stratification for polynomials: The open stratum

In this paper we consider the Newton polygons of L-functions coming from additive exponential sums associated to a polynomial over a finite field Fq. These polygons define a stratification of the space of polynomials of fixed degree. We determine the open stratum: we give the generic Newton polygon for polynomials of degree d?2 when the characteristic p?3d, and the Hasse polynomial over Fp, i.e. the equation defining the hypersurface complementary to the open stratum.