“…The papers by Faria and Röst [12], Freedman and Ruan [15], Garay and Hofbauer [16], Hetzer and Shen [20], Hirsch et al [21], Langa et al [26], Magal and Zhao [28], Mierczyński and Shen [30], Mierczyński et al [31], Novo et al [36], Salceanu and Smith [43], Schreiber [44,45], Thieme [51,52], Wang and Zhao [54], and references therein, provide a long but not complete list of works on this topic.…”