POSITIVE SUBHARMONIC SOLUTIONS TO SUPERLINEAR ODES WITH INDEFINITE WEIGHT.

We study the positive subharmonic solutions to the second order nonlinear ordinary differential equation ... where g(u) has superlinear growth both at zero and at infinity, and q(t) is a T-periodic sign-changing weight. Under the sharp mean value condition f₀^T q(t) dt < 0, combining Mawhin's coincidence degree theory with the Poincare-Birkhoff fixed point theorem, we prove that there exist positive subharmonic solutions of order k for any large integer k. Moreover, when the negative part of q(t) is sufficiently large, using a topological approach still based on coincidence degree theory, we obtain the existence of positive subharmonics of order k for any integer k ≥ 2. [ABSTRACT FROM AUTHOR]