Operator Models via Compact Embedding

Abstract

In this paper, we extend several function-theoretic and geometric constructions to the realm of multi-variable operator theory. A commuting tuple of bounded linear operators on a Hilbert space, equipped with a cyclic vector, is abbreviated as a cyclic commuting tuple. We encode the complete information of such a tuple into a single positive compact operator on the Fock space. Drawing parallels with spectral geometry, we investigate how the spectral data of this positive compact operator -- its eigenvalues and eigenfunction -- reflect fundamental properties of the operator tuple. Our main contributions are threefold. First, we establish a Weyl-type approximation formula for certain operator tuples, demonstrating that the asymptotic behavior of eigenvalues carries geometric information about the vanishing variety. Second, we construct two kernel functions derived from the eigenvalues and eigenfunctions: the first extends the Bergman kernel, while the second extends the Fourier-Laplace transformation. We prove that the Fourier-Laplace kernel defines a reproducing kernel Hilbert space (RKHS) on which the coordinate differential operators are unitarily equivalent to the adjoint tuple. Consequently, we show that the convergence points of the Bergman-type kernel characterize the joint eigenvalues of the adjoint tuple. Finally, we obtain a Paley-Wiener-Schwartz type theorem for cyclic commuting tuples, characterizing cyclic commuting tuples whose associated Agler's linear functional are distributions. For tuples consisting of matrices, we obtains a more explicit characterization.