Abstract. In this paper, we precise the asymptotic behaviour of Newton polygons of L-functions associated to character sums, coming from certain n variable Laurent polynomials. In order to do this, we use the free sum on convex polytopes. This operation allows the determination of the limit of generic Newton polygons for the sum ∆ = ∆ 1 ⊕ ∆ 2 when we know the limit of generic Newton polygons for each factor. To our knowledge, these are the first results concerning the asymptotic behaviour of Newton polygons for multivariable polynomials when the generic Newton polygon differs from the combinatorial (Hodge) polygon associated to the polyhedron.
IntroductionIn the following, we note k a finite field with q = p a elements, and k r its degree r extension in an algebraic closure k fixed once and for all. Let x = (x 1 , . . . , x n ) be an n-tuple of indeterminates, and f (x) = i∈Z n a i x i ∈ k[x, x −1 ] be a Laurent polynomial in n variables with coefficients in k. If ψ denotes a non trivial additive character of k, let ψ r := ψ • Tr kr /k be the character induced by ψ on k r ; let χ be a multiplicative character of (k × ) n , and χ r := χ • N kr/k its extension to (k × r ) n . From f , ψ and χ, we form the character sums over each extension of kthen from these sums we define the L-functionWhen χ is trivial, we simply denote this function by L(f ; T ). From the works of Dwork and Grothendieck (cf.[8], [10]) we know that it is rational.Let us begin with a trivial χ; this is the most classical case in the literature. The first result about the L-function is due to Deligne [6, Théorème 8.4]. For a polynomial of degree d prime to p, whose higher degree form defines a nonsingular hypersurface in the projective space P n−1 , the function L ′ (f, T ) (here the sums are defined over A n , not over G n m ) has degree (d − 1) n . More generally, one can associate to the polynomial f its Newton polyhedron at infinity, which is the convex polyhedron ∆ defined in affine space R n as the convex 1991 Mathematics Subject Classification. 11M38,13A02,52B20.