Theorem inf5

Description: The statement "there exists a set that is a proper subset of its union" is equivalent to the Axiom of Infinity (see theorem infeq5 ). This provides us with a very compact way to express the Axiom of Infinity using only elementary symbols. (Contributed by NM, 3-Jun-2005)

	Ref	Expression
Assertion	inf5	⊢ ∃ 𝑥 𝑥 ⊊ ∪ 𝑥

Proof

Step	Hyp	Ref	Expression
1		omex	⊢ ω ∈ V
2		infeq5i	⊢ ( ω ∈ V → ∃ 𝑥 𝑥 ⊊ ∪ 𝑥 )
3	1 2	ax-mp	⊢ ∃ 𝑥 𝑥 ⊊ ∪ 𝑥