 The geometric minimum action method: A least action principle on the space of curves by Matthias Heymann and Eric Vanden-Eijnden

By Matthias Heymann, Eric Vanden-Eijnden.

Communications on Pure and Applied Mathematics 61, 1052–1117. DOI: doi.org/10.1002/cpa.20238 . Author-hosted PDF. Sections 1.1–1.3: action admissibility, noise convention, free-time quasipotential and geometric reformulation. Wiki uses drift a and covariance D to avoid the paper's a/b notation ambiguity; the time-reparameterization and gradient examples are explicitly derived.