curr goto 관계,relation#s-5
https://everything2.com/title/Equivalence relation (Good)
https://proofwiki.org/wiki/Definition:Equivalence_Relation
https://proofwiki.org/wiki/Definition:Equivalence_Relation
동치관계
https://artofproblemsolving.com/wiki/index.php/Equivalence_relation
https://en.citizendium.org/wiki/Equivalence_relation
https://encyclopediaofmath.org/wiki/Equivalence_relation
https://artofproblemsolving.com/wiki/index.php/Equivalence_relation
https://en.citizendium.org/wiki/Equivalence_relation
https://encyclopediaofmath.org/wiki/Equivalence_relation