同値關係
equivalence relation
對稱律$ x\sim y\supset y\sim x 推移律$ (x\sim y\land y\sim z)\supset x\sim z 記號$ =\equiv\approx\cong\simeq\Doteq\risingdotseq\iff:=\coloneqq\colonequals\sim\backsim
元$ a_{\in S}を代表元 (representative) とする同値類$ \lbrack a\rbrack:=\{x|x\in S,a\sim x\} 商集合
$ S/\sim:=\{\lbrack x\rbrack|x\in S\}
商寫像
$ \pi:S\twoheadrightarrow S/\sim,x\mapsto\lbrack x\rbrack
類別 (classification。分割 (partition))