Chiappinelli, R., Mawhin, J., & Nugari, R. (1987). Generalized Ambrosetti-prodi Conditions for Nonlinear 2-point Boundary-value-problems. Journal of Differential Equations, 69(3), 422-434. https://doi.org/10.1016/0022-0396(87)90127-6 (Original work published 1987)