文摘
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, for all \({\alpha > 0}\) , the singular values \({s_{n}}\) of \({\Gamma}\) satisfy the bound \({s_{n}= O(n^{-\alpha})}\) as \({n \to \infty}\) provided \({h(j)=O(j^{-1}(\log j )^{-\alpha})}\) as \({j\to \infty}\) . These estimates on \({s_{n}}\) are sharp in the power scale of \({\alpha}\) . Similar results are obtained for Hankel operators \({{{\bf \Gamma}}}\) realized in \({L^2({\mathbb{R}}_+)}\) as integral operators with kernels \({\mathbf{h}(t+s)}\) . In this case the estimates of singular values of \({{{\bf \Gamma}}}\) are determined by the behavior of \({\mathbf{h}(t)}\) as \({t \to 0}\) and as \({t \to \infty}\) .