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Multiple eigenvalues of the Laplace-Beltrami operator and deformation of the Riemannian metric
Given a connected compact $C^\infty$ manifold M of dimension $n\ge2$ without
boundary, a Riemannian metric $g$ on M and an eigenvalue $\lambda^*(M,g)$ of multiplicity
$\nu\ge2$ of the Laplace-Beltrami operator $\Delta_g,$ we provide a sufficient
condition such that the set of the deformations of the metric $g,$ which preserve the
multiplicity of the eigenvalue, is locally a manifold of codimension $1/2 \nu(\nu+1)-1$ in
the space of $C^k$ symmetric covariant 2-tensors on M. Furthermore we prove that such a condition
is fulfilled when $n=2$ and $\nu=2.$