Let X = G/H be a reductive symmetric space with rank G/H = rank K/K ∩ H, where K (resp.K ∩ H) is a maximal compact subgroup of G (resp. of H). We investigate the discrete spectrum of certain Clifford-Klein forms Γ\X, where Γ is a discrete subgroup of G acting properly discontinuously and freely on X: we construct an infinite set of joint eigenvalues for ''intrisic'' differential operators on Γ\X, and this set is stable under small deformations of Γ in G.
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© Toshiyuki Kobayashi