An eigenvalue problem related to the non-linear sigma-model: analytical and numerical results

An eigenvalue problem relevant for the non-linear sigma model with singular metric is considered. We prove the existence of a non-degenerate pure point spectrum for all finite values of the size R of the system. In the infrared (IR) regime (large R) the eigenvalues admit a power series expansion around the IR critical point R --> infinity. We compute high order coefficients and prove that the series converges for all finite values of R. In the ultraviolet (UV) limit the spectrum condenses into a continuum spectrum with a set of residual bound states. The spectrum agrees nicely with the central charge computed by the thermodynamic Bethe ansatz method.