文摘
Given a function \(f\) on the positive half-line \({\mathbb R}_+\) and a sequence (finite or infinite) of points \(X=\{x_k\}_{k=1}^\omega \) in \({\mathbb R}^n\), we define and study matrices \({\mathcal S}_X(f)=[f(\Vert x_i-x_j\Vert )]_{i,j=1}^\omega \) called Schoenberg’s matrices. We are primarily interested in those matrices which generate bounded and invertible linear operators \(S_X(f)\) on \(\ell ^2({\mathbb N})\). We provide conditions on \(X\) and \(f\) for the latter to hold. If \(f\) is an \(\ell ^2\)-positive definite function, such conditions are given in terms of the Schoenberg measure \(\sigma _f\). Examples of Schoenberg’s operators with various spectral properties are presented. We also approach Schoenberg’s matrices from the viewpoint of harmonic analysis on \({\mathbb R}^n\), wherein the notion of the strong \(X\)-positive definiteness plays a key role. In particular, we prove that each radial \(\ell ^2\) -positive definite function is strongly \(X\) -positive definite whenever \(X\) is a separated set. We also implement a “grammization-procedure for certain positive definite Schoenberg’s matrices. This leads to Riesz–Fischer and Riesz sequences (Riesz bases in their linear span) of the form \({\mathcal F}_X(g)=\{g(\cdot -x_j)\}_{x_j\in X}\) for certain radial functions \(g\in L^2({\mathbb R}^n)\). Keywords Infinite matrices Schur test Riesz sequences Completely monotone functions Fourier transform Gram matrices Minimal sequences Toeplitz operators