[1] H. AMANN A. AMBROSETTI G. MANCINI: 
Elliptic equations with noninvertible Fredholm linear part and bounded nonlinearities. Math. Z. 158, 179-194 (1978). 
MR 0481498 
[3] S. FUČÍK: 
Solvability of Nonlinear Equations and Boundary Value Problems. to appear in B. Riedel Publishing Company, Holland. 
MR 0620638 
[4] P. HESS B. RUF: 
On a superlinear elliptic boundary value problem. Math. Z. 164, 9-14 (1978). 
MR 0514603 
[5] P. J. McKENNA J. RAUCH: 
Strongly nonlinear perturbations of nonnegative boundary value problems with kernel. Journ. of Diff. Equations 28, 253-265 (1978). 
MR 0491053