[1] H. Amann: A note on degree theory for gradient mappings. Proc. A.M.S. 85 (1982), 591 to 595. MR 0660610 | Zbl 0501.58012

[2] H. Amann G. Mancini: Some applications of monotone operator theory to resonance problems. Nonlin. Anal. T.M.A. Vol. 3, n. 6 (1979), 825-830. MR 0548954

[3] A. Ambrosetti K. Mancini: Theorems of existence and multiplicity for nonlinear elliptic problems with noninvertible linear part. Аnnali Sc. Nor. Sup. Pisa, Vol. V, n. 1 (1978), 15-28. MR 0487001

[4] A. Ambrosetti G. Mancini: Existence and multiplicity results for nonlinear elliptic problems with linear part at гesonance. The case of the simple eigenvalue. J. Diff. Eq., Vol. 28, n. 2 (1978), 220-245. MR 0492839

[5] P. W. Bates A. Castro: Necessary and sufficient conditions for existence of solutions to equations with noninvertible linear part. Revista Colomb. de Mat., Vol. XV (1981), 7-24. MR 0674931

[6] J. V. A. Concalves: On bounded nonlinear perturbations of an elliptic equation at resonance. Nonlin. Аnalysis T.M.А. Vol. I, n. 5 (1981), 57-60. MR 0597281

[7] P. Drábek: Bounded nonlinear perturbations of second order linear elliptic problems. Comment. Math. Univ. Caгoliae 22, 2 (1981), 215-221. MR 0620358

[8] P. Drábek: Solvability of nonlinear problems at resonance. Comment. Math. Univ. Carolinae 25, 2 (1982), 359-368. MR 0664981

[9] S. Fučik M. Krbec: Boundary value problems with bounded nonlinearity and general nullspace of the linear part. Math. Z. 155 (1977), 129-138. MR 0473513

[10] P. Hess: А гemark on the preceding paper of Fučík and Krbec. Math. Z. 155 (1977), 139 to 141. MR 0473514

[11] D. G. de Figueiredo, Wei-Ming Ni: Perturbations of second order linear elliptic problems by nonlinearities without Landesman-Lazer condition. Nonlin. Аnalysis T.M.А., Vol. 3, n. 5 (1979), 629-634. MR 0541873 | Zbl 0429.35035

[12] E. G. Sitnikova: Theorem on a strong zero for an elliptic equation of high order (Russian). Mat. Ѕbornik 8 (123), n. 3 (1970), 376-397. MR 0264219