Definition(Ordered Pairs (Kazimierz Kuratowski;1921)) (a,b)≌{a,{a,b} 4口，1①，43，t夏，30Q0 Hengeng Wei (thfweionjn.edu.cn 1-9 Set Theory (II):Relations 2019 1203 7/51. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Definition (Ordered Pairs (Kazimierz Kuratowski; 1921)) (a, b) ≜ { {a}, {a, b} } Theorem (a, b) = (c, d) ⇐⇒ a = c ∧ b = d Proof. { {a}, {a, b} } = { {c}, {c, d} } Case I : a = b Case II : a ̸= b Hengfeng Wei (hfwei@nju.edu.cn) 1-9 Set Theory (II): Relations 2019 年 12 月 03 日 7 / 51