1. V. A. Kondrat’ev and E. M. Landis, “Qualitative theory of second-order linear partial differential equations,” in Partial Differential Equations–3, Itogi Nauki i Tekhniki. Ser. Sovrem. Probl. Mat. Fund. Napr. (VINITI, Moscow, 1988), Vol. 32, pp. 99–215 [in Russian].
2. D. Gilbarg and N. Trudinger, Elliptic Partial Differential Equations of Second Order (Springer- Verlag, Berlin–New York, 1983).
3. M. A. Vorontsov, N. G. Iroshnikov, and R. I. Abernathy, “Diffractive patterns in a nonlinear optical the two-dimensional feedback system with field rotation,” Chaos Solitons Fractals 4 (8-9), 1701–1716 (1994).
4. A. L. Skubachevskii, “On the Hopf bifurcation for a quasilinear parabolic functional- differential equation,” Differ. Equ. 34 (10), 1395–1402 (1998).
5. A. L. Skubachevskii, “Bifurcation periodic solutions for nonlinear parabolic functional- differential equations arising in optoelectronics,” Nonlinear Anal. 32 (2), 261–278 (1998).