“…The case of the sequence of the k-th eigenfunction is considered in Theorem 3.8. This kind of issue, namely the approximation of the solutions of problems set on cylinders by the solution of problems set on their section, is addressed in [2], [3], [4], [5], [6], [7], [8] for some differential equations, variational inequalities or systems. If one wants to normalize u k in such a way that its L 2 -norm is equal to one, one is led to choose A k such that Suppose now that we want to find the eigenvalues and eigenfunctions of the problem…”