1. I. V. Skrypnik, Higher-Order Elliptic Equations [in Russian], Naukova Dumka, Kiev (1973).
2. O. A. Ladyzhenskaya and N. N. Ural'tseva, Linear and Quasilinear Elliptic Equations [in Russian], Nauka, Moscow (1973).
3. A. M. Dement'eva, V. M. Krasnosel'skii, and M. A. Krasnosel'skii, ?Degenerate critical points of functions of several variables,? Proceedings of the Seminar on Differential Equations, Kuibyshev, No. 5, 23?31 (1980).
4. N. A. Bobylev and M. A. Krasnosel'skii, Testing for Extremum. Degenerate Cases [in Russian], Preprint of the Institute for Control Problems, USSR Academy of Sciences, Moscow (1981).
5. N. A. Bobylev, ?On a deformational method for investigating the performance functionals of systems with an infinite number of degrees of freedom,? Avtomat. Telemekh., No. 7, 11?18 (1981).