On singular limits of finite Hilbert transform operators on multi‐intervals

Abstract

AbstractInthis paper, we study the small‐λ spectral asymptotics of an integral operator defined on two multi‐intervals J and E, when the multi‐intervals touch each other (but their interiors are disjoint). The operator is closely related to the multi‐interval finite Hilbert transform (FHT). This case can be viewed as a singular limit of self‐adjoint Hilbert–Schmidt integral operators with so‐called integrable kernels, where the limiting operator is still bounded, but has a continuous spectral component. The regular case when , and is of the Hilbert–Schmidt class, was studied in an earlier paper by the authors. The main assumption in this paper is that is a single interval (although part of our analysis is valid in a more general situation). We show that the eigenvalues of , if they exist, do not accumulate at . Combined with the results in an earlier paper by the authors, this implies that , the subspace of discontinuity (the span of all eigenfunctions) of , is finite dimensional and consists of functions that are smooth in the interiors of J and E. We also obtain an approximation to the kernel of the unitary transformation that diagonalizes , and obtain a precise estimate of the exponential instability of inverting . Our work is based on the method of Riemann–Hilbert problem and the nonlinear steepest descent method of Deift and Zhou.