“…The importance of TWS makes many people devote themselves to the study of its existence with the help of various methods. Some well-known methods include the Schauder's fixed-point theorem [11,22,25,27,34,35,39,41,40], monotone iteration technique [36,37,42], shooting method [12,13,14,23], Conley index [19,20] and so on. However, there is another useful but little noticed approach, namely, the geometric singular perturbation (GSP) theory [18,24], which is mainly suitable for systems with small parameters [31].…”