“…This method originated in the dynamic programming theory and was initially applied by Bellman and Kalaba [8]. A systematic development of the method to ordinary differential equations has been provided by Lakshmikantham and Vatsala [18], and there are some generalized results of the method to various types of differential systems, we can refer to the monographs [16,17], for functional differential equations [4,12], for impulsive equations [3,6], for partial differential equations [5,9,14,24], for others [21,22,27] and references cited therein. However, there were few applicable results of the method to singular differential systems [1,13,19,23].…”