圖論當中,英文cycle)係指一條唔係空嘅路徑,而條路徑裏面淨係得最頭嗰個同最尾嗰個頂點係重複咗嘅[1]

呢幅圖當中嘅 H-D-G-H(紅色線)係一個環。

  1. Balakrishnan, V. K. (2005). Schaum's outline of theory and problems of graph theory ([Nachdr.] ed.). McGraw–Hill.