We find and discuss asymptotic formulas for orthonormal polynomials $P_{ n}(z)$ with recurrence coefficients $a_{ n}, b_{ n}$. Our main goal is to consider the case where off-diagonal elements $a_{ n}\to\infty$ as $n\to\infty$. Formulas obtained are essentially different for relatively small and large diagonal elements $b_{ n}$. Our analysis is intimately linked with spectral theory of Jacobi operators $J$ with coefficients $a_{ n}, b_{ n}$ and a study of the corresponding second order difference equations. We introduce the Jost solutions $f_{ n}(z)$, $n\geq -1$, of such equations by a condition for $n\to\infty$ and suggest an Ansatz for them playing the role of the semiclassical Liouville-Green Ansatz for solutions of the Schr\"odinger equation. This allows us to study the spectral structure of Jacobi operators and their eigenfunctions $P_{ n}(z)$ by traditional methods of spectral theory developed for differential equations. In particular, we express all coefficients in asymptotic formulas for $P_{ n}(z)$ as $n \to\infty$ in terms of the Wronskian of the solutions $ P_{ n} (z) $ and $ f_{ n} (z)$. The formulas obtained for $P_{ n}(z)$ generalize the asymptotic formulas for the classical Hermite polynomials where $a_{ n}=\sqrt{ (n+1)/2}$ and $b_{ n}=0$.
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