Abstract
In this article we study the operator which appears in the Usawa algorithm classically used in the solution of the Stokes problem, in particular when finite element discretisations are used (see [1],[11]). We call this operator the Usawa operator. We consider S (and-Δ) as a self–adojoint operator in L2Ωfor several domains Ωwith a particular emphasis on the channel flow problem; in this case we determine all the eigenvalues of S and give a precise description of S.