Notes:The foundations of Mathematics - Kunen

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Chapter I: Set Theory

Section I.1

Section I.2: The axioms

# Axiom Definition Comment
0 Existence [ilmath]\exists x(x=x)[/ilmath]
1 Extensionality [ilmath]\forall z(x\in x\leftrightarrow z\in y)\rightarrow x=y[/ilmath]
2 Foundation [ilmath]\exists y(y\in x)\rightarrow\exists y(y\in x\wedge\not\exists z(z\in x\wedge z\in y))[/ilmath]
3 Comprehension schema [ilmath]\exists y\forall x(x\in y\leftrightarrow x\in z\wedge\varphi(x))[/ilmath] [ilmath]\varphi[/ilmath] a formula, [ilmath]y[/ilmath] not free
4 Pairing [ilmath]\exists z(x\in z\wedge y\in z)[/ilmath]
5 Union [ilmath]\exists A\forall Y\forall x(x\in Y\wedge Y\in\mathcal{F}\rightarrow x\in A)[/ilmath] Union of [ilmath]\mathcal{F} [/ilmath]
6 Replacement schema [ilmath]\forall x\in A\exists!y\varphi(x,y)\rightarrow\exists B\forall x\in A\exists y\in B\varphi(x,y)[/ilmath] For each formula, without [ilmath]B[/ilmath] free
7 Infinity [ilmath]\exists x(\emptyset\in x\wedge\forall y\in x(S(y)\in x))[/ilmath]
8 Power set [ilmath]\exists y\forall z(z\subseteq x\rightarrow z\in y)[/ilmath]
9 Choice [ilmath]\emptyset\not\in F\wedge\forall x\in F\forall y\in F(x\neq y\rightarrow x\cap y=\emptyset)\rightarrow \exists C\forall x\in F(\text{Sing}(C\cap x))[/ilmath]
Alec's note: "axiom" 0 can be shown from the axiom of infinity.

Theories

Theory Axioms Comment
1 2 3 4 5 6 7 8 9
ZFC x x x x x x x x x
ZF x x x x x x x x
ZC x x x x x x x x
Z x x x x x x x
Z- x x x x x x
ZF- x x x x x x x
ZC- x x x x x x x
ZFC- x x x x x x x x