A discrete Smorodinsky--Winternitz II superintegrable system

Abstract

We construct a discrete Smorodinsky--Winternitz II superintegrable system on a triangular region of the two-dimensional square lattice. The model is built from a pair of commuting finite-difference number operators with finite spectrum together with an associated ladder-operator structure. We show that it is maximally superintegrable and that its symmetry algebra admits a Hahn-algebra presentation. The spectral problem is solved exactly in terms of bivariate orthogonal polynomials of mixed Krawtchouk and dual Hahn type associated with the factorized $A_2$-Leonard pair. Finally, we show that the continuum limit recovers the continuous Smorodinsky--Winternitz II system together with its Hermite--Laguerre eigenfunctions. We further explain how the Hahn presentation of the discrete symmetry algebra becomes singular in this limit, while the limiting algebraic structure is naturally described by the Laguerre--Heun algebra associated with Cartesian and parabolic separation of variables.