Given a compact Riemannian manifold (M, g) and two positive functions ρ
and σ, we are interested in the eigenvalues of the Dirichlet energy
functional weighted by σ, with respect to the L 2 inner product weighted
by ρ. Under some regularity conditions on ρ and σ, these
eigenvalues are those of the operator ρ^{-1} div(σ∇u) with
Neumann conditions on the boundary if ∂M = ∅. We investigate
the effect of the weights on eigenvalues and discuss the existence of lower and
upper bounds under the condition that the total mass is preserved