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Published online by Cambridge University Press: 10 December 2009
We analyse isospectral sets of potentials associated to a given ‘generalized periodic’ boundary condition
in SL(2, R) for the Sturm-Liouville equation on the unit interval. This is done by first studying the larger manifold M of all pairs of boundary conditions and potentials with a given spectrum and characterizing the critical points of the map from M to the trace a + d Isospectral sets appear as slices of M whose geometry is determined by the critical point structure of the trace function. This paper completes the classification of isospectral sets for all real self-adjoint boundary conditions.
With an appendix by Johan Tysk, Department of Mathematics, University of California, Los Angeles, CA 90024, USA.