On bounded solutions for second order linear differential equations with   negative stiffness
  Second order linear non-autonomous differential equations with negative stiffness are considered. Using Chetaev-like (Lyapunov-like) functions, necessary (sufficient) conditions are found for the solutions to be bounded for all initial conditions if any one of the coefficients is constant. Conclusions are then extended to include systems where both coefficients are time-varying. 
C.A. Terrero-Escalante