Bernatskaya, J. N. “Perturbation Method for a Parabolic Equation With Drift on a Riemannian Manifold”. Ukrains’kyi Matematychnyi Zhurnal, Vol. 56, no. 2, Feb. 2004, pp. 148-59, https://umj.imath.kiev.ua/index.php/umj/article/view/3739.