We investigate the set V f of horizontal critical points of a polynomial function f for the standard Engel structure defined by the 1-forms ω 3 = dx 3 − x 1 dx 2 and ω 4 = dx 4 − x 3 dx 2 , endowed with the sub-Riemannian metric g SR = dx 2 1 + dx 2 2 . For a generic polynomial, we show that the set f of points in V f , where V f is not transverse to the Engel distribution, does not have a connected component which is contained in a fiber of f . Then, we prove that each trajectory of the horizontal gradient of f approaching the set V f has a limit.Keywords Engel structure · Horizontal gradient · Horizontal curve · Limit of trajectories Mathematics Subject Classifications (2010) 14P10 · 53C17 · 58A30 · 58Kxx
IntroductionAn Engel structure is a non-integrable distribution of rank 2 on a four-dimensional manifold which satisfies the following conditions:where [., .] denotes the Lie bracket. Engel structures are stable (or generic) in the sense that all C 2 -small perturbation of an Engel structure is still an Engel structure. A manifold with an Engel structure is called an Engel manifold. In this paper, we will deal with the standard