Asymptotic behaviour of eigenvalues of Hankel operators

Abstract: We consider compact Hankel operators realized in $ \ell^2(\mathbb Z_+)$ as infinite matrices $\Gamma$ with matrix elements $h(j+k)$. Roughly speaking, we show that if $h(j)\sim (b_{1}+ (-1)^j b_{-1}) j^{-1}(\log j)^{-\alpha}$ as $j\to \infty$ for some $\alpha>0$, then the eigenvalues of $\Gamma$ satisfy $\lambda_{n}^{\pm} (\Gamma)\sim c^{\pm} n^{-\alpha}$ as $n\to \infty$. The asymptotic coefficients $c^{\pm}$ are explicitly expressed in terms of the asymptotic coefficients $b_{1} $ and $b_{-1}$. Similar results are obtained for Hankel operators $\mathbf \Gamma$ realized in $ L^2(\mathbb R_+)$ as integral operators with kernels $\mathbf h(t+s)$. In this case the asymptotics of eigenvalues $\lambda_{n}^{\pm} (\mathbf \Gamma)$ are determined by the behaviour of $\mathbf h(t)$ as $t\to 0$ and as $t\to \infty$.