Periodic Boundary Value Problems for Second-Order Functional Differential Equations

Upper and lower solution method plays an important role in studying boundary value problems for nonlinear differential equations; see 1 and the references therein. Recently, many authors are devoted to extend its applications to boundary value problems of functional differential equations 2–5 . Suppose α is one upper solution or lower solution of periodic boundary value problems for second-order differential equation; the condition α 0 α T is required. A neutral problem is that whether we can define upper and lower solution without assuming α 0 α T . The aim of the present paper is to discuss the following second order functional differential equation