On the discrete spectrum of a family of differential operators

A family $\BA_\a$ of differential operators depending on a real parameter $\a$ is considered. The problem can be formulated in the language of perturbation theory of quadratic forms. The perturbation is only relatively bounded but not relatively compact with respect to the unperturbed form. The spectral properties of the operator $\BA_\a$ strongly depend on $\a$. In particular, for $\a<\sqrt2$ the spectrum of $\BA_\a$ below 1/2 is finite, while for $\a>\sqrt2$ the operator has no eigenvalues at all. We study the asymptotic behaviour of the number of eigenvalues as $\a\nearrow\sqrt2$. We reduce this problem to the one on the spectral asymptotics for a certain Jacobi matrix.