文摘
In this paper, we construct nonconstant lower and upper solutions for the periodic boundary value problem \(x'+f(t,x)=e(t)\), \(x(0)=x(T)\) and find their estimates. We prove the existence of positive solutions for the singular problem \(x'+g(x)=e(t)\), \(x(0)=x(T)\) by using these results.