Remarks on J. J. Nieto's paper: Nonlinear second-order periodic boundary value problems
The authors prove a theorem improving earlier results of \textit{J. J. Nieto} [J. Math. Anal. Appl. 130, 22-29 (1988; Zbl 0678.34022)] on second order periodic boundary value problems (1) \(-u''=f(t,u),u(0)=u(2\pi)\), \(u'(0)=u'(2\pi)\) where f is continuous in (t,u) and decreasing in u for each fixed t, assuming the existence of an upper and lower solution for (1).
second order periodic boundary value problems
scientific article; zbMATH DE number 4173392