On positive Jacobi matrices with compact inverses

Abstract: We consider positive Jacobi matrices $J$ with compact inverses and consequently with purely discrete spectra. A number of properties of the corresponding sequence of orthogonal polynomials is studied including the convergence of their zeros, the vague convergence of the zero counting measures and of the Christoffel--Darboux kernels on the diagonal. Particularly, if the inverse of $J$ belongs to some Schatten class, we identify the asymptotic behaviour of the sequence of orthogonal polynomials and express it in terms of its regularized characteristic function. In the even more special case when the inverse of $J$ belongs to the trace class we derive various formulas for the orthogonality measure, eigenvectors of $J$ as well as for the functions of the second kind and related objects. These general results are given a more explicit form in the case when $-J$ is a generator of a Birth--Death process. Among others we provide a formula for the trace of the inverse of $J$. We illustrate our results by introducing and studying a modification of $q$-Laguerre polynomials corresponding to a determinate moment problem.