E. R. Avakov, A. A. Agrachev, A. V. Arutyunov, “The level set of a smooth mapping in a neighborhood of a singular point, and zeros of a quadratic mapping”, Mathematics of the USSR-Sbornik, 1992, 73:2, 455–466
The level set M" /> of a smooth mapping F" /> in a neighborhood of an anormal point is investigated. Concepts of 2-regularity are introduced for it. It is proved that if the mapping is 2-regular at the point under consideration, then in a neighborhood of it the set M" /> is locally diffeomorphic to the set of zeros of the second differential of F" />.