A rigorous mathematical construction of Feynman path integrals for the Schr\"odinger equation with magnetic field

A Feynman path integral formula for the Schr"odinger equation with magnetic field is rigorously mathematically realized in terms of infinite dimensional oscillatory integrals. We show (by the example of a linear vector potential) that the requirement of the independence of the integral on the approximation procedure forces the introduction of a counterterm to be added to the classical action functional. This provides a natural explanation for the appearance of a Stratonovich integral in the path integral formula for both the Schr"odinger and heat equation with magnetic field.