About sign-constancy of Green's functions for impulsive second order delay equations

We consider the following second order differential equation with delay [formula] In this paper we find necessary and sufficient conditions of positivity of Green's functions for this impulsive equation coupled with one or two-point boundary conditions in the form of theorems about differential inequalities. By choosing the test function in these theorems, we obtain simple sufficient conditions. For example, the inequality [formula] is a basic one, implying negativity of Green's function of two-point problem for this impulsive equation in the case 0<γi≤1, 0<δi≤1 for i=1,…,p.