Axiom ax-dc 10452

Description: Dependent Choice. Axiom DC1 of [Schechter] p. 149. This theorem is weaker than the Axiom of Choice but is stronger than Countable Choice. It shows the existence of a sequence whose values can only be shown to exist (but cannot be constructed explicitly) and also depend on earlier values in the sequence. Dependent choice is equivalent to the statement that every (nonempty) pruned tree has a branch. This axiom is redundant in ZFC; see axdc 10527. But ZF+DC is strictly weaker than ZF+AC, so this axiom provides for theorems that do not need the full power of AC. (Contributed by Mario Carneiro, 25-Jan-2013.)

Assertion

Ref	Expression
ax-dc	⊢ ((∃𝑦∃𝑧 𝑦𝑥𝑧 ∧ ran 𝑥 ⊆ dom 𝑥) → ∃𝑓∀𝑛 ∈ ω (𝑓‘𝑛)𝑥(𝑓‘suc 𝑛))

Distinct variable group:   𝑓,𝑛,𝑥,𝑦,𝑧

Detailed syntax breakdown of Axiom ax-dc

Step	Hyp	Ref	Expression
1		vy	. . . . . . 7 setvar 𝑦
2	1	cv 1569	. . . . . 6 class 𝑦
3		vz	. . . . . . 7 setvar 𝑧
4	3	cv 1569	. . . . . 6 class 𝑧
5		vx	. . . . . . 7 setvar 𝑥
6	5	cv 1569	. . . . . 6 class 𝑥
7	2, 4, 6	wbr 5107	. . . . 5 wff 𝑦𝑥𝑧
8	7, 3	wex 1812	. . . 4 wff ∃𝑧 𝑦𝑥𝑧
9	8, 1	wex 1812	. . 3 wff ∃𝑦∃𝑧 𝑦𝑥𝑧
10	6	crn 5660	. . . 4 class ran 𝑥
11	6	cdm 5659	. . . 4 class dom 𝑥
12	10, 11	wss 3902	. . 3 wff ran 𝑥 ⊆ dom 𝑥
13	9, 12	wa 401	. 2 wff (∃𝑦∃𝑧 𝑦𝑥𝑧 ∧ ran 𝑥 ⊆ dom 𝑥)
14		vn	. . . . . . 7 setvar 𝑛
15	14	cv 1569	. . . . . 6 class 𝑛
16		vf	. . . . . . 7 setvar 𝑓
17	16	cv 1569	. . . . . 6 class 𝑓
18	15, 17	cfv 6537	. . . . 5 class (𝑓‘𝑛)
19	15	csuc 6363	. . . . . 6 class suc 𝑛
20	19, 17	cfv 6537	. . . . 5 class (𝑓‘suc 𝑛)
21	18, 20, 6	wbr 5107	. . . 4 wff (𝑓‘𝑛)𝑥(𝑓‘suc 𝑛)
22		com 7866	. . . 4 class ω
23	21, 14, 22	wral 3078	. . 3 wff ∀𝑛 ∈ ω (𝑓‘𝑛)𝑥(𝑓‘suc 𝑛)
24	23, 16	wex 1812	. 2 wff ∃𝑓∀𝑛 ∈ ω (𝑓‘𝑛)𝑥(𝑓‘suc 𝑛)
25	13, 24	wi 4	1 wff ((∃𝑦∃𝑧 𝑦𝑥𝑧 ∧ ran 𝑥 ⊆ dom 𝑥) → ∃𝑓∀𝑛 ∈ ω (𝑓‘𝑛)𝑥(𝑓‘suc 𝑛))

This axiom is used by:  dcomex  10453  axdc2lem  10454