“…Thirdly, our results can be applied to both strong singularity and weak singularity. In fact, following Lazer and Solimini's work, there are many good papers appearing in this field by different kinds of methods such as the fixed point theorems [8,9,27], lower and upper solution [1,22,23], Leray-Schauder alternative principle [12,28,32] and coincidence degree theory [5,29,37], and so on. Different from above methods, in this paper, we use the theory of the fixed point in cones to study the periodic solution.…”