Landesman Lazer type results for first order periodic problems Donal O'Regan (1997) Commentationes Mathematicae Universitatis Carolinae Existence of nonnegative solutions are established for the periodic problem y ' = f ( t , y ) a.eȯn [ 0 , T ] , y ( 0 ) = y ( T ) . Here the nonlinearity f satisfies a Landesman Lazer type condition.