Abstract. In this paper, we improve results of Gillot, Kumar and Moreno to estimate some exponential sums by means of q-degrees. The method consists in applying suitable elementary transformations to see an exponential sum over a finite field as an exponential sum over a product of subfields in order to apply Deligne bound. In particular, we obtain new results on the spectral amplitude of some monomials.
Introduction.Exponential sums and bounds for them are exploited by coding theorists and communications engineers [15]. The minimal distance of dual BCH and other cyclic codes can be evaluated in terms of exponential sums. They are also useful in the study of sequences with small correlations for spread-spectrum and other communication applications. In both cases, the estimation of exponential sums is often a key point for the construction of a good code. In this paper, we focus on the estimation of exponential sums over finite fields for some polynomials. First, we begin introducing the tools and known results about spectral amplitude. Then, we generalise results of Kumar and Moreno [10] over spectral amplitude of some monomials in odd characteristics. We define the q-degree and the principle of multivariate point of view to be able to apply the results of Deligne [4] for exponential sums over a product of finite fields. Thus, we obtain a bound in terms of q-degree generalising the result of Gillot [6].