Periodic solutions to second order nonautonomous differential systems with gyroscopic forces

Keywords: Second order differential systems Saddle point theorem Generalized Ahmad–Lazer–Paul type condition a b s t r a c t Existence of periodic solutions to second order differential systems with gyroscopic forces is considered via variational methods, where a generalized Ahmad–Lazer–Paul type condition is used. We do not impose the condition that the gyroscopic forces are small.