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Metamath Proof Explorer

Theorem 4nn

Description: 4 is a positive integer. (Contributed by NM, 8-Jan-2006)

		Ref	Expression
	Assertion	4nn	$⊢ 4 \in ℕ$

Step	Hyp	Ref	Expression
1		df-4	$⊢ 4 = 3 + 1$
2		3nn	$⊢ 3 \in ℕ$
3		peano2nn	$⊢ 3 \in ℕ \to 3 + 1 \in ℕ$
4	2 3	ax-mp	$⊢ 3 + 1 \in ℕ$
5	1 4	eqeltri	$⊢ 4 \in ℕ$