Theorem logbcl

Description: General logarithm closure. (Contributed by David A. Wheeler, 17-Jul-2017)

	Ref	Expression
Assertion	logbcl	⊢ ( ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ∧ 𝑋 ∈ ( ℂ ∖ { 0 } ) ) → ( 𝐵 logb 𝑋 ) ∈ ℂ )

Proof

Step	Hyp	Ref	Expression
1		logbval	⊢ ( ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ∧ 𝑋 ∈ ( ℂ ∖ { 0 } ) ) → ( 𝐵 logb 𝑋 ) = ( ( log ‘ 𝑋 ) / ( log ‘ 𝐵 ) ) )
2		eldifsn	⊢ ( 𝑋 ∈ ( ℂ ∖ { 0 } ) ↔ ( 𝑋 ∈ ℂ ∧ 𝑋 ≠ 0 ) )
3		logcl	⊢ ( ( 𝑋 ∈ ℂ ∧ 𝑋 ≠ 0 ) → ( log ‘ 𝑋 ) ∈ ℂ )
4	2 3	sylbi	⊢ ( 𝑋 ∈ ( ℂ ∖ { 0 } ) → ( log ‘ 𝑋 ) ∈ ℂ )
5	4	adantl	⊢ ( ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ∧ 𝑋 ∈ ( ℂ ∖ { 0 } ) ) → ( log ‘ 𝑋 ) ∈ ℂ )
6		eldifi	⊢ ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) → 𝐵 ∈ ℂ )
7		eldifpr	⊢ ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ↔ ( 𝐵 ∈ ℂ ∧ 𝐵 ≠ 0 ∧ 𝐵 ≠ 1 ) )
8	7	simp2bi	⊢ ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) → 𝐵 ≠ 0 )
9	6 8	logcld	⊢ ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) → ( log ‘ 𝐵 ) ∈ ℂ )
10	9	adantr	⊢ ( ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ∧ 𝑋 ∈ ( ℂ ∖ { 0 } ) ) → ( log ‘ 𝐵 ) ∈ ℂ )
11		logccne0	⊢ ( ( 𝐵 ∈ ℂ ∧ 𝐵 ≠ 0 ∧ 𝐵 ≠ 1 ) → ( log ‘ 𝐵 ) ≠ 0 )
12	7 11	sylbi	⊢ ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) → ( log ‘ 𝐵 ) ≠ 0 )
13	12	adantr	⊢ ( ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ∧ 𝑋 ∈ ( ℂ ∖ { 0 } ) ) → ( log ‘ 𝐵 ) ≠ 0 )
14	5 10 13	divcld	⊢ ( ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ∧ 𝑋 ∈ ( ℂ ∖ { 0 } ) ) → ( ( log ‘ 𝑋 ) / ( log ‘ 𝐵 ) ) ∈ ℂ )
15	1 14	eqeltrd	⊢ ( ( 𝐵 ∈ ( ℂ ∖ { 0 , 1 } ) ∧ 𝑋 ∈ ( ℂ ∖ { 0 } ) ) → ( 𝐵 logb 𝑋 ) ∈ ℂ )