Parasyuk, I. O. “Quasiperiodic Extremals of Nonautonomous Lagrangian Systems on Riemannian Manifolds”. Ukrains’kyi Matematychnyi Zhurnal, vol. 66, no. 10, Oct. 2014, pp. 1387–1406, https://umj.imath.kiev.ua/index.php/umj/article/view/2230.