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On the Discrete Spectrum of a Family of Differential Operators
M. Z. Solomyak Weizmann Institute of Science
Abstract:
We consider a family
$\mathbf{A}_\alpha$ of differential operators in
$L^2(\mathbb{R}^2)$ depending on a parameter
$\alpha\ge0$. The operator
$\mathbf{A}_\alpha$ formally corresponds to the quadratic form
$$
\mathbf{a}_\alpha[U]=\int_{\mathbb{R}^2}\biggl(|U_x|^2+\frac{1}{2}(|U_y|^2 +y^2|U|^2)\biggr)\,dx\,dy
+\alpha\int_\mathbb{R}y|U(0,y)|^2\,dy.
$$
The perturbation determined by the second term in this sum is only relatively bounded but not
relatively compact with respect to the unperturbed quadratic form
$\mathbf{a}_0$.
The spectral properties of
$\mathbf{A}_\alpha$ strongly depend on
$\alpha$. In particular,
$\sigma(\mathbf{A}_0)=[1/2,\infty)$; for
$0<\alpha<\sqrt 2$, finitely many eigenvalues
$l_n<1/2$ are added to the spectrum; and for
$\alpha>\sqrt2$ (where the quadratic form approach does not apply), the spectrum is purely continuous and coincides with
$\mathbb{R}$. We study the asymptotic behavior of the number of eigenvalues as
$\alpha\nearrow\sqrt 2$ and reduce this problem to the problem on the spectral asymptotics for a certain Jacobi matrix.
Keywords:
discrete spectrum, perturbation, Jacobi matrix.
UDC:
517.97 Received: 30.01.2004
DOI:
10.4213/faa118