Krasnoselskii, AM., Krasnoselskii, MA., Mawhin, J., & Pokrovskii, A. (1994). Generalized Guiding Functions in a Problem On High-frequency Forced-oscillations. Nonlinear Analysis: Theory, Methods & Applications, 22(11), 1357-1371. https://doi.org/10.1016/0362-546X(94)90117-1 (Original work published 1994)