Axiom ax-dc 10368

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 10443. 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 1541	. . . . . 6 class 𝑦
3		vz	. . . . . . 7 setvar 𝑧
4	3	cv 1541	. . . . . 6 class 𝑧
5		vx	. . . . . . 7 setvar 𝑥
6	5	cv 1541	. . . . . 6 class 𝑥
7	2, 4, 6	wbr 5100	. . . . 5 wff 𝑦𝑥𝑧
8	7, 3	wex 1781	. . . 4 wff ∃𝑧 𝑦𝑥𝑧
9	8, 1	wex 1781	. . 3 wff ∃𝑦∃𝑧 𝑦𝑥𝑧
10	6	crn 5633	. . . 4 class ran 𝑥
11	6	cdm 5632	. . . 4 class dom 𝑥
12	10, 11	wss 3903	. . 3 wff ran 𝑥 ⊆ dom 𝑥
13	9, 12	wa 395	. 2 wff (∃𝑦∃𝑧 𝑦𝑥𝑧 ∧ ran 𝑥 ⊆ dom 𝑥)
14		vn	. . . . . . 7 setvar 𝑛
15	14	cv 1541	. . . . . 6 class 𝑛
16		vf	. . . . . . 7 setvar 𝑓
17	16	cv 1541	. . . . . 6 class 𝑓
18	15, 17	cfv 6500	. . . . 5 class (𝑓‘𝑛)
19	15	csuc 6327	. . . . . 6 class suc 𝑛
20	19, 17	cfv 6500	. . . . 5 class (𝑓‘suc 𝑛)
21	18, 20, 6	wbr 5100	. . . 4 wff (𝑓‘𝑛)𝑥(𝑓‘suc 𝑛)
22		com 7818	. . . 4 class ω
23	21, 14, 22	wral 3052	. . 3 wff ∀𝑛 ∈ ω (𝑓‘𝑛)𝑥(𝑓‘suc 𝑛)
24	23, 16	wex 1781	. 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 referenced by:  dcomex  10369  axdc2lem  10370