Feynman's operational calculi: Spectral theory for noncommuting self-adjoint operators

The spectral theorem for commuting self-adjoint operators along with the associated functional (or operational) calculus is among the most useful and beautiful results of analysis. It is well known that forming a functional calculus for noncommuting self-adjoint operators is far more problematic. The central result of this paper establishes a rich functional calculus for any finite number of noncommuting (i.e. not necessarily commuting) bounded, self-adjoint operators A ₁,..., A _n and associated continuous Borel probability measures μ ₁, ⋯, μ _n on [0,1]. Fix A ₁,..., A _n . Then each choice of an n-tuple (μ₁,...,μ_n) of measures determines one of Feynman's operational calculi acting on a certain Banach algebra of analytic functions even when A ₁, ..., A _n are just bounded linear operators on a Banach space. The Hilbert space setting along with self-adjointness allows us to extend the operational calculi well beyond the analytic functions. Using results and ideas drawn largely from the proof of our main theorem, we also establish a family of Trotter product type formulas suitable for Feynman's operational calculi.