Extreme eigenvalues of an integral operator

Abstract

We study the family of compact operators $B_{\alpha} = V A_{\alpha} V$, $\alpha>0$ in $L^2(\mathbb R^d)$, $d\ge 1$, where $A_{\alpha}$ is the pseudo-differential operator with symbol $a_{\alpha}(\boldsymbol\xi) = a(\alpha\boldsymbol\xi)$, and both functions $a$ and $V$ are real-valued and decay at infinity. We assume that $a$ and $V$ attain their maximal values $A_0>0$, $V_0>0$, only at $\boldsymbol\xi = \mathbf 0$ and $\mathbf x = \mathbf 0$. We also assume that a(\boldsymbol\xi) = &\ A_0 - \Psi_{\gamma}(\boldsymbol\xi) + o(|\boldsymbol\xi|^{\gamma}),\ |\boldsymbol\xi|\to 0, V(\mathbf x) = &\ V_0 - \Phi_{\beta}(\mathbf x) + o(|\mathbf x|^{\beta}),\ |\mathbf x|\to 0, with some functions $\Psi_{\gamma}(\boldsymbol\xi)>0$, $\boldsymbol\xi\not =\mathbf 0$ and $\Phi_{\beta}(\mathbf x) >0$, $\mathbf x\not = \mathbf 0$ that are homogeneous of degree $\gamma>0$ and $\beta >0$ respectively. The main result is the following asymptotic formula for the eigenvalues $\lambda_{\alpha}^{(n)}$ of the operator $B_{\alpha}$ (arranged in descending order counting multiplicity) for fixed $n$ and $\alpha\to 0$: \lambda_{\alpha}^{(n)} = A_0V_0^2 - \mu^{(n)} \alpha^{\sigma} + o(\alpha^{\sigma}), \alpha\to 0, where $\sigma^{-1} = \gamma^{-1}+ \beta^{-1}$, and $\mu^{(n)}$ are the eigenvalues (arranged in ascending order counting multiplicity) of the model operator $T$ with symbol $V_0^2\Psi_{\gamma}(\boldsymbol\xi) + 2A_0 V_0 \Phi_{\beta}(\mathbf x)$.