[1] A. C. Lazer, P. J. McKenna:
A semi-Fredholm principle for periodically forced systems with homogeneous nonlinearities. Proc. Amer. Math. Soc., 106(1989), 119-125.
MR 0942635 |
Zbl 0683.34027
[2] J. Mawhin:
Topological Degree Methods in Nonlinear Boundary Value Problems. CBMS Conference in Math. n°. 40, Americal Mathematical Soc., Providence, Rhode Island, 1979.
MR 0525202 |
Zbl 0414.34025