Exotic B-series representation of the Feller semigroup for Itô diffusions and the MSR path integral

Abstract: In this paper we consider the expansion of the Feller semigroup of a one-dimensional It^o diffusion as a power series in time. Taking our moves from previous results on expansions labelled by exotic trees, we derive an explicit expression for the combinatorial factors involved, that leads to an exotic Butcher series representation. A key step is the extension of the notion of tree factorial and Connes-Moscovici weight to this richer family of rooted trees. The ensuing expression is suitable for a comparison with the perturbative path integral construction of the statistics of the diffusion, known in the literature as Martin-Siggia-Rose formalism. Resorting to multi-indices to represent pre-Feynman diagrams, we show that the latter coincides with the exotic B-series representation of the semigroup, giving it a solid mathematical foundation.