I. G. Tsar'kov, “Smoothness of Solutions of the Eikonal Equation and Regular Points of Their Level Surfaces”, Russ. J. Math. Phys., 30:2 (2023), 259
I. G. Tsar'kov, “Geometry of the Singular Set of Hypersurfaces and the Eikonal Equation”, Russ. J. Math. Phys., 29:2 (2022), 240
И. Г. Царьков, “Геометрия особого множества гиперповерхностей и уравнение эйконала”, Матем. заметки, 108:3 (2020), 441–451 ; I. G. Tsar'kov, “The Geometry of a Singular Set of Hypersurfaces and the Eikonal Equation”, Math. Notes, 108:3 (2020), 426–433