“…Some classical tools have been widely used to get the solutions of impulsive differential equations, such as fixed point theorems in cones, topological degree theory (including continuation method and coincidence degree theory), the method of lower and upper solutions, and the critical point theory. For the theory and classical results, we refer the readers to the references, [4], [17], [19], [26], [32] and books [2], [22], [31].…”