[1] M. S. BERGER M.  SCHECHTER: 
On the solvability of semilinear operator equations and elliptic boundary value problems. Bull. Amer. Math. Soc. 78 (1972), 741-745. 
MR 0303374[2] R. COURANT D. HILBERT: Methods of Mathematical Physics. Vol. 1, New York, 1953.
[3] S. FUČÍK: 
Fredholm alternative for nonlinear operators in Banach spaces and its applications to differential and integral equations. Čas. pěst. mat. 96 (1971), 371-390. 
MR 0326502 [4J S. FUČÍK: 
Nonlinear equations with noninvertible linear part. Czechoslovak Math. Journal (to appear). 
MR 0348568[5] S. FUČÍK M. KUČERA J. NEČAS: 
Ranges of nonlinear asymptotically linear operators. J. Diff. Equations (to appear). 
MR 0372696[6] S. FUČÍK J. NEČAS J. SOUČEK V. SOUČEK: 
Spectral Analysis of Nonlinear Operators. Lecture Notes in Mathematics No 346, Springer Verlag 1973. 
MR 0467421[8] M. A. KRASNOSELSKIJ: Positive Solutions of Operator's Equations. (Russian), Moscow 1962.
[9] E. M. LANDESMAN A. C. LAZER: 
Nonlinear perturbations of linear boundary value problems at resonance. Journ. Math. Mech. 19 (1970), 609-623. 
MR 0267269[10] J. NEČAS: 
Fredholm alternative for nonlinear operators with application to partial differential equations and integral equations. Čas. pěst. mat. 97 (1972), 65-71. 
MR 0308882[11] J. NEČAS: 
On the range of nonlinear operators with linear asymptotes which are not invertible. Comment. Math. Univ. Carolinae 14 (1973), 63-72. 
MR 0318995[12] L. NIRENBERG: 
An application of generalized degree to a class of nonlinear problems. Troisième Colloq. Anal. Fonct. Liège Centre Beige de Recherches Mathématiques, 1971, pp. 57-73. 
MR 0413207 | 
Zbl 0317.35036[13] L. NIRENBERG: 
Generalized degree and nonlinear problems. "Contributions to Nonlinear Analysis" edited by E. Zarantonello. Academic Press 1971, pp. 1-9. 
MR 0388188 | 
Zbl 0267.47034[15] S. A. WILLIAMS: 
A sharp sufficient condition for solution of a nonlinear elliptic boundary value problem. J. Diff. Equations 8 (1970), 580-586. 
MR 0267267 | 
Zbl 0209.13003