Asymptotics of a section by a plane of the sub-riemannian sphere on the Engel group near abnormal trajectory

In this paper we compute an asymptotics of the curve that is the intersection of the unit sub-Riemannian sphere on the Engel group with the subspace \{𝑥 = = 0\} near the abnormal trajectory. It follows from the asymptotics that this curve is not analytic at the point (1, 0, 0, 0). (In Russian)