Abstract: | In this paper, we will prove the existence of infinitely many harmonic and subharmonic solutions for the second order differential
equation ẍ + g(x) = f(t, x) using the phase plane analysis methods and Poincaré–Birkhoff Theorem, where the nonlinear restoring field g exhibits superlinear conditions near the infinity and strong singularity at the origin, and f(t, x) = a(t)x
γ + b(t, x) where 0 ≤ γ ≤ 1 and b(t, x) is bounded. |