Langouche, F., Roekaerts, D., & Tirapegui, E. (1981). Wkb-type Expansion for Langevin-equations .2. Covariant Expansion. Physica A: Statistical Mechanics and its Applications, 108(1), 221-232. https://doi.org/10.1016/0378-4371(81)90176-X (Original work published 1981)