5 5 9 9 1 3 1 3 … 3 3 7 7 1 1 1 1 …
The expression above can be expressed in the form
exp ( A π [ ln ( π [ Γ ( C B ) ] n ) + γ ] )
where A , B , C , and n are integers with g cd ( B , C ) = 1 , with B and C positive. Determine A + B + C + n .
Notations:
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Let us concatenate the product as follows, naming the product P .
P = k = 1 ∏ ∞ ( 2 k + 1 ) ( − 1 ) k + 1 / ( 2 k + 1 )
If we'll take the logarithm of both sides, we'll get
ln P = k = 1 ∑ ∞ ( − 1 ) k + 1 2 k + 1 ln ( 2 k + 1 )
The Malmsten-Kummer series expansion for γ is as follows:
γ = ln π − 4 ln ( Γ ( 4 3 ) ) + π 4 ⋅ k = 1 ∑ ∞ ( − 1 ) k + 1 2 k + 1 ln ( 2 k + 1 )
From here we can now see that
k = 1 ∑ ∞ ( − 1 ) k + 1 2 k + 1 ln ( 2 k + 1 ) = 4 π [ ln ( π [ Γ ( 4 3 ) ] 4 ) + γ ]
Giving us 4 + 3 + 4 + 4 = 1 5 .
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Consider the Dirichlet beta function β ( x ) = n = 0 ∑ ∞ ( 2 n + 1 ) x ( − 1 ) n Then β ′ ( x ) = n = 0 ∑ ∞ ( 2 n + 1 ) x ( − 1 ) n + 1 ln ( 2 n + 1 ) and hence β ′ ( 1 ) = n = 0 ∑ ∞ 2 n + 1 ( − 1 ) n + 1 ln ( 2 n + 1 ) and so the infinite product we want is e β ′ ( 1 ) = e x p [ 4 1 π ( 2 ln 2 + 3 ln π − 4 ln Γ ( 4 1 ) + γ ) ] = e x p [ 4 1 π [ ln ( Γ ( 4 1 ) 4 4 π 3 ) + γ ] ] = e x p [ 4 1 π [ ln ( π Γ ( 4 3 ) 4 ) + γ ] ] making the answer 4 + 3 + 4 + 4 = 1 5 .