00331nam a2200121Ia 4500082001900000100002500019245006000044260004900104300000600153490004000159500000500199700000500204  a530.1bL495.CT  aLefschetz, S. d ed.  aContributions to the theory of nonlinear oscillationsb  aNew JerseybPrinceton University Pressc1958  a   aAnnals of Mathematics studies v 41  a  a