Dualité et Spectres des (a, b)-modules

In the first part of this paper, we introduce the notion of δ-dual of a quasi-unipotent (a, b)-module and prove a formula for its spectrum. In the second part, we show that the (a, b)-module associated with an isolated hypersurface singularity of dimension n is (n + 1)-selfdual. From these two independant facts, we conclude with the classical symmetry property of the spectrum of the singularity.