Langouche, F., Roekaerts, D., & Tirapegui, E. (1980). Covariant Wkb Expansion in Riemannian Space. Physics Letters. Section A: General, Atomic and Solid State Physics, 77(1), 3-6. https://doi.org/10.1016/0375-9601(80)90615-5 (Original work published 1980)