Finite dimensional approximations for Hilbert space operators and applications in Quantum Mechanics
In this work, we develop a unified framework for quasidiagonal and Følner-type approximations of linear operators on Hilbert spaces. These approximations (originally formulated for bounded operators and operator algebras) involve sequences of non-zero finite rank orthogonal projections that asymptotically commute with the operator -- either in norm (quasidiagonal) or in mean (Følner). Such structures guarantee spectral approximation results in terms of their finite sections. We extend this theory to unbounded, densely defined closable operators, establishing a generalization of Halmos' classical result: every closable quasidiagonal operator is a compact perturbation of a closable block-diagonal operator on the same domain. Likewise, we introduce sparse Følner sequences and establish an interplay between quasidiagonal approximations and the existence of sparse Følner sequences. The theoretical developments are illustrated with explicit examples using different types of weighted shifts and applied to quantum mechanical models, including a detailed treatment of the Weyl algebra and its Schrödinger representation.