[A] AMANN H.: On the unique solvability of semi-linear operator equatгons in Hilbert spaces. J. Math. Pures Appl. 61 (1982), 149-175. MR 0673303

[AZ] AMANN H.-ZEHNDER E.: Nontrгvial solutions for a class of nonresonance problems and applications to nonlinear differential equations. Ann. Scuola Norm. Sup. Pisa Cl. Sci. (4) VII (1980), 539-603. MR 0600524

[H] HOFER H.: Variational and topological methods in partially ordered Hilbert spaces. Math. Ann. 261 (1982), 493-514. MR 0682663 | Zbl 0488.47034

[QŽ] QUITTNER P.-ŽUBRINIC D.: On the unique solvability of nonresonant elliptic equations. Comment. Math. Univ. Carolin. 27 (1986), 301-306. MR 0857550 | Zbl 0603.35035

[VK] VOEVODIN V. V.-KUZNECOV, YU. A.: Matrices and Calculations. [Matricy i vychislenia], Nauka, Moscow, 1977. (Russian)