良擬序,按「嵌入」排序,如此組成良擬序當且僅當 ( X , ≤ ) {\displaystyle (X,\leq )} 本身是良擬序,此結論稱為希格曼引理(英语:Higman's lemma)。其中所謂字串 u {\displaystyle u} 可以嵌入到 v {\displaystyle v} ,意思是 v {\displaystyle
自由阿贝尔群Mathematik, 1994, 1994 (451): 175–219, MR 1277300, S2CID 118116330, doi:10.1515/crll.1994.451.175 , proof of Lemma 2.3: "the trivial group is the direct product