a1 Department of Mathematical Sciences, Tsinghua University, Beijing 100084, China
a2 Department of Mathematics, Northeast Normal University, Changchun 130024, Jilin, China
a3 Department of Mathematical Science, Florida Institute of Technology, Melbourne, FL 32901–6975, United States of America
a4 Department of Mathematics, Northeast Normal University, Changchun 130024, Jilin, China
a5 Department of Mathematics, National University of Ireland, Galway, Ireland
In this paper, we study the existence of positive periodic solutions to the equation x″ = f (t, x). It is proved that such a equation has more than one positive periodic solution when the nonlinearity changes sign. The proof relies on a fixed point theorem in cones.
(Received August 30 2005)