Singular-value (and eigenvalue) distribution and Krylov preconditioning of sequences of sampling matrices approximating integral operators

K̃ is a compact operator and therefore its spectrum forms a bounded sequence having zero as unique accumulation point. Here we first consider in detail the approximation of K̃ by using rectangle formula in the case where Ω = [0, 1] and the step is h = 1/n. The related linear application can be represented as a matrix An of size n. In accordance with the compact character of the continuous operator, we prove that {An} ∼σ 0 and {An} ∼λ 0, i.e., the considered sequence has singular values and eigenvalues clustered at zero. Moreover the cluster is strong in perfect analogy with the compactness of K̃. Several generalizations are sketched, with special attention to the general case of pure sampling sequences, and few examples and numerical experiments are critically discussed, including the use of GMRES and preconditioned GMRES for large linear systems coming from the numerical approximation of integral equations of the form