Spectral theory of Jacobi operators with increasing coefficients. The critical case

Spectral properties of Jacobi operators $J$ are intimately related to an asymptotic behavior of the corresponding orthogonal polynomials $P_{n}(z)$ as $n\to\infty$. We study the case where the off-diagonal coefficients $a_{n}$ and, eventually, diagonal coefficients $b_{n}$ of $J$ tend to infinity in such a way that the ratio $\gamma_{n}:=2^{-1}b_{n} (a_{n}a_{n-1})^{-1/2}$ has a finite limit $\gamma$. %We study an asymptotic behavior as $n\to\infty$ of the orthogonal polynomials $P_{n}(z)$ defined by Jacobi recurrence coefficients $a_{n}$ (off-diagonal terms) and $b_{n}$ (diagonal terms). %We consider the case $a_{n}\to\infty$ and suppose that the sequence $\gamma_{n}:=2^{-1}b_{n} (a_{n}a_{n-1})^{-1/2}$ has a limit $\gamma$ as $n\to\infty$. In the case $|\gamma | < 1$ asymptotic formulas for $P_{n}(z)$ generalize those for the Hermite polynomials and the corresponding Jacobi operators $J$ have absolutely continuous spectra covering the whole real line. If $|\gamma | > 1$, then spectra of the operators $J$ are discrete. Our goal is to investigate the critical case $| \gamma |=1$ that occurs, for example, for the Laguerre polynomials. The formulas obtained depend crucially on the rate of growth of the coefficients $a_{n}$ (or $b_{n}$) and are qualitatively different in the cases where $a_{n}\to \infty$ faster or slower then $n$. For the fast growth of $a_{n}$, we also have to distinguish the cases $|\gamma_{n}| \to 1-0$ and $|\gamma_{n}| \to 1+0$. Spectral properties of the corresponding Jacobi operators are quite different in all these cases. Our approach works for an arbitrary power growth of the Jacobi coefficients.