Axiom (Ordered Pairs) (a,b)=(c,d→a=c∧b=d 4口，1①，43，t夏，30Q0 Hengfeng Wei (bhfweionjn.edu.cn 1-9 Set Theory (II):Relations 2019 12 03 6/51. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Axiom (Ordered Pairs) (a, b) = (c, d) ⇐⇒ a = c ∧ b = d Definition (Ordered Pairs (Kazimierz Kuratowski; 1921)) (a, b) ≜ { {a}, {a, b} } Hengfeng Wei (hfwei@nju.edu.cn) 1-9 Set Theory (II): Relations 2019 年 12 月 03 日 6 / 51