Theorem stirling

Description: Stirling's approximation formula for n factorial. The proof follows two major steps: first it is proven that S and n factorial are asymptotically equivalent, up to an unknown constant. Then, using Wallis' formula for π it is proven that the unknown constant is the square root of π and then the exact Stirling's formula is established. This is Metamath 100 proof #90. (Contributed by Glauco Siliprandi, 29-Jun-2017)

	Ref	Expression
Hypothesis	stirling.1	⊢ 𝑆 = ( 𝑛 ∈ ℕ0 ↦ ( ( √ ‘ ( ( 2 · π ) · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) )
Assertion	stirling	⊢ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( 𝑆 ‘ 𝑛 ) ) ) ⇝ 1

Proof

Step	Hyp	Ref	Expression
1		stirling.1	⊢ 𝑆 = ( 𝑛 ∈ ℕ0 ↦ ( ( √ ‘ ( ( 2 · π ) · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) )
2		eqid	⊢ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) = ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) )
3		eqid	⊢ ( 𝑛 ∈ ℕ ↦ ( log ‘ ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ 𝑛 ) ) ) = ( 𝑛 ∈ ℕ ↦ ( log ‘ ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ 𝑛 ) ) )
4	2 3	stirlinglem14	⊢ ∃ 𝑐 ∈ ℝ+ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐
5		nfv	⊢ Ⅎ 𝑛 𝑐 ∈ ℝ+
6		nfmpt1	⊢ Ⅎ 𝑛 ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) )
7		nfcv	⊢ Ⅎ 𝑛 ⇝
8		nfcv	⊢ Ⅎ 𝑛 𝑐
9	6 7 8	nfbr	⊢ Ⅎ 𝑛 ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐
10	5 9	nfan	⊢ Ⅎ 𝑛 ( 𝑐 ∈ ℝ+ ∧ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐 )
11		eqid	⊢ ( 𝑛 ∈ ℕ ↦ ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ ( 2 · 𝑛 ) ) ) = ( 𝑛 ∈ ℕ ↦ ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ ( 2 · 𝑛 ) ) )
12		eqid	⊢ ( 𝑛 ∈ ℕ ↦ ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) = ( 𝑛 ∈ ℕ ↦ ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) )
13		eqid	⊢ ( 𝑛 ∈ ℕ ↦ ( ( ( ( 2 ↑ ( 4 · 𝑛 ) ) · ( ( ! ‘ 𝑛 ) ↑ 4 ) ) / ( ( ! ‘ ( 2 · 𝑛 ) ) ↑ 2 ) ) / ( ( 2 · 𝑛 ) + 1 ) ) ) = ( 𝑛 ∈ ℕ ↦ ( ( ( ( 2 ↑ ( 4 · 𝑛 ) ) · ( ( ! ‘ 𝑛 ) ↑ 4 ) ) / ( ( ! ‘ ( 2 · 𝑛 ) ) ↑ 2 ) ) / ( ( 2 · 𝑛 ) + 1 ) ) )
14		eqid	⊢ ( 𝑛 ∈ ℕ ↦ ( ( ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ 𝑛 ) ↑ 4 ) / ( ( ( 𝑛 ∈ ℕ ↦ ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ ( 2 · 𝑛 ) ) ) ‘ 𝑛 ) ↑ 2 ) ) ) = ( 𝑛 ∈ ℕ ↦ ( ( ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ 𝑛 ) ↑ 4 ) / ( ( ( 𝑛 ∈ ℕ ↦ ( ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ‘ ( 2 · 𝑛 ) ) ) ‘ 𝑛 ) ↑ 2 ) ) )
15		eqid	⊢ ( 𝑛 ∈ ℕ ↦ ( ( 𝑛 ↑ 2 ) / ( 𝑛 · ( ( 2 · 𝑛 ) + 1 ) ) ) ) = ( 𝑛 ∈ ℕ ↦ ( ( 𝑛 ↑ 2 ) / ( 𝑛 · ( ( 2 · 𝑛 ) + 1 ) ) ) )
16		simpl	⊢ ( ( 𝑐 ∈ ℝ+ ∧ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐 ) → 𝑐 ∈ ℝ+ )
17		simpr	⊢ ( ( 𝑐 ∈ ℝ+ ∧ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐 ) → ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐 )
18	10 1 2 11 12 13 14 15 16 17	stirlinglem15	⊢ ( ( 𝑐 ∈ ℝ+ ∧ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐 ) → ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( 𝑆 ‘ 𝑛 ) ) ) ⇝ 1 )
19	18	rexlimiva	⊢ ( ∃ 𝑐 ∈ ℝ+ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( ( √ ‘ ( 2 · 𝑛 ) ) · ( ( 𝑛 / e ) ↑ 𝑛 ) ) ) ) ⇝ 𝑐 → ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( 𝑆 ‘ 𝑛 ) ) ) ⇝ 1 )
20	4 19	ax-mp	⊢ ( 𝑛 ∈ ℕ ↦ ( ( ! ‘ 𝑛 ) / ( 𝑆 ‘ 𝑛 ) ) ) ⇝ 1