“…When dim span E = 2, Theorem 4.15 is just a restatement of Theorem 4.13 because we can take F = E. However, this sufficient condition is also true when dim span E = 3 so this theorem improves the aforementioned one. It also extends previous results of [17], which do not consider Dirichlet boundary conditions, and of [3], [9], where a non-constructive proof was obtained only for the case dim span F = 2.…”