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Jouni Parkkonen; Frédéric Paulin
On the closedness of approximation spectra
Journal de théorie des nombres de Bordeaux, 21 no. 3 (2009), p. 703-712
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Résumé - Abstract

Generalizing Cusick’s theorem on the closedness of the classical Lagrange spectrum for the approximation of real numbers by rational ones, we prove that various approximation spectra are closed, using penetration properties of the geodesic flow in cusp neighbourhoods in negatively curved manifolds and a result of Maucourant [Mau].

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