Two approaches to the use of unbounded operators in feynman’s operational calculus

In this paper, we investigate two approaches to the use of unbounded operators in Feynman’s operational calculus. The first involves using a functional calculus for unbounded operators introduced by A. E. Taylor in the paper [34]. The second approach uses analytic families of closed unbounded operators as discussed in [19]. For each approach, we discuss the essential properties of the operational calculus as well as continuity (or stability) properties. Finally, for the approach using the Taylor calculus, we discussion a connection between Feynman’s operational calculus in this setting with the Modified Feynman Integral of M. L. Lapidus ([14, 20]).