## On second order periodic boundary-value problems with upper and lower solutions in the reversed order.(English)Zbl 1096.34010

Summary: We study the differential equation with the periodic boundary value $u''(t)=f(t, u(t), u'(t)),\quad t\in [0, 2\pi],\quad u(0)=u(2\pi), \quad u'(0)=u'(2\pi).$ The existence of solutions to the periodic boundary problem with appropriate conditions is proved by using an upper and lower solution method.

### MSC:

 34B15 Nonlinear boundary value problems for ordinary differential equations
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