Degenerate normal modes from permutation symmetry
= Degenerate normal modes from permutation symmetry
If the mass and stiffness matrices commute with a <permutation representation>, each symmetry subspace is invariant under the <generalized eigenvalue problem for small oscillations>. On an irreducible subspace, <Schur lemma> forces a commuting operator to be scalar, producing a degenerate <normal mode> multiplet.