Let M be a compact connected Riemannian manifold and j the Laplace-Beltrami operator acting on the space of all complex-valued smooth functions on M.We denote by Spec(M)the set of all eigenvalues of ⊿, repeated according to multiplicity. For any real number r∈R,we denote by Spec(M)r the set {λ∈ Spec (M)