1. L. L. Dines, “494–498,” Bull. Amer.Math. Soc. 47 (1941).
2. A. S. Matveev, “On the convexity of the images of quadratic mappings,” Algebra i Analiz 10 (2), 159–196 (1998) [St. PetersburgMath. J. 10 (2), 343–372 (1998)].
3. B. T. Polyak, “Convexity of quadratic transformations and its use in control and optimization,” J. Optim. Theory Appl. 99 (3), 553–583 (1998).
4. A. A. Agrachev, “Topology of quadratic mappings and hessians of smooth mappings,” in Itogi Nauki i Tekhniki, Collection: Algebra. Topology. Geometry (VINITI, Moscow, 1988), Vol. 26, pp. 85–124. [J. Sov. Math. 49 (3), 990–1013 (1990)].
5. A. V. Arutyunov, “The implicit function theorem as a realization of the Lagrange principle. Abnormal points,” Mat. Sb. 191 (1), 3–26 (2000) [Sb.Math. 191 (1–2), 1–24 (2000)].