Theorem zfinf 8419

Description: Axiom of Infinity expressed with the fewest number of different variables. (New usage is discouraged.) (Contributed by NM, 14-Aug-2003.)

Assertion

Ref	Expression
zfinf	⊢ ∃𝑥(𝑦 ∈ 𝑥 ∧ ∀𝑦(𝑦 ∈ 𝑥 → ∃𝑧(𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)))

Distinct variable group:   𝑥,𝑦,𝑧

Proof of Theorem zfinf

Dummy variable 𝑤 is distinct from all other variables.

Step	Hyp	Ref	Expression
1		ax-inf 8418	. 2 ⊢ ∃𝑥(𝑦 ∈ 𝑥 ∧ ∀𝑤(𝑤 ∈ 𝑥 → ∃𝑧(𝑤 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)))
2		elequ1 1984	. . . . . 6 ⊢ (𝑤 = 𝑦 → (𝑤 ∈ 𝑥 ↔ 𝑦 ∈ 𝑥))
3		elequ1 1984	. . . . . . . 8 ⊢ (𝑤 = 𝑦 → (𝑤 ∈ 𝑧 ↔ 𝑦 ∈ 𝑧))
4	3	anbi1d 737	. . . . . . 7 ⊢ (𝑤 = 𝑦 → ((𝑤 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥) ↔ (𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)))
5	4	exbidv 1837	. . . . . 6 ⊢ (𝑤 = 𝑦 → (∃𝑧(𝑤 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥) ↔ ∃𝑧(𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)))
6	2, 5	imbi12d 333	. . . . 5 ⊢ (𝑤 = 𝑦 → ((𝑤 ∈ 𝑥 → ∃𝑧(𝑤 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)) ↔ (𝑦 ∈ 𝑥 → ∃𝑧(𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥))))
7	6	cbvalv 2261	. . . 4 ⊢ (∀𝑤(𝑤 ∈ 𝑥 → ∃𝑧(𝑤 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)) ↔ ∀𝑦(𝑦 ∈ 𝑥 → ∃𝑧(𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)))
8	7	anbi2i 726	. . 3 ⊢ ((𝑦 ∈ 𝑥 ∧ ∀𝑤(𝑤 ∈ 𝑥 → ∃𝑧(𝑤 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥))) ↔ (𝑦 ∈ 𝑥 ∧ ∀𝑦(𝑦 ∈ 𝑥 → ∃𝑧(𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥))))
9	8	exbii 1764	. 2 ⊢ (∃𝑥(𝑦 ∈ 𝑥 ∧ ∀𝑤(𝑤 ∈ 𝑥 → ∃𝑧(𝑤 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥))) ↔ ∃𝑥(𝑦 ∈ 𝑥 ∧ ∀𝑦(𝑦 ∈ 𝑥 → ∃𝑧(𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥))))
10	1, 9	mpbi 219	1 ⊢ ∃𝑥(𝑦 ∈ 𝑥 ∧ ∀𝑦(𝑦 ∈ 𝑥 → ∃𝑧(𝑦 ∈ 𝑧 ∧ 𝑧 ∈ 𝑥)))

Syntax hints:   → wi 4   ∧ wa 383  ∀wal 1473  ∃wex 1695

This theorem was proved from axioms:  ax-mp 5  ax-1 6  ax-2 7  ax-3 8  ax-gen 1713  ax-4 1728  ax-5 1827  ax-6 1875  ax-7 1922  ax-8 1979  ax-10 2006  ax-11 2021  ax-12 2034  ax-13 2234  ax-inf 8418

This theorem depends on definitions:  df-bi 196  df-an 385  df-ex 1696  df-nf 1701

This theorem is referenced by:  axinf2  8420  axinfndlem1  9306