k = 1 ∑ 2 0 1 8 ∫ 0 ∞ sin 2 k ( x 1 ) d x = a π ⋅ Γ ( d ) Γ ( c b )
The equation above holds true for positive integers a , b , c and d with g cd ( b , c ) = 1 . Find a + b + c + d .
Notation:
Γ
(
⋅
)
denotes the
Gamma function
.
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Wow. I see now why my approach at summing up the even numbered Wallis Integrals miraculously worked. Thank you for showing that!
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One way would be to integrate z 2 e i a z − 1 around a semicircular contour in the upper half place, indented at the origin to avoid the pole.
Sir could you please explain how you got \begin{aligned} \int_0^\infty \sin^{2k}\tfrac{1}{x}\,dx & = \int_0^\infty \frac{\sin^{2k}x}{x^2}\,dx
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Sin^(2k)(1/x) as( (sin^(2k)(x))/x^(2))
Use the substitution x ↦ x − 1 .
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Start with the integral ∫ 0 ∞ x 2 1 − cos a x = 2 1 π a a > 0 Using the Binomial Theorem, for any positive integer k , sin 2 k x = ( 2 i e i x − e − i x ) k = 2 2 k ( − 1 ) k j = 0 ∑ 2 k ( − 1 ) j ( j 2 k ) e 2 ( k − j ) i x = 2 2 k − 1 1 j = 1 ∑ k ( − 1 ) j − 1 ( k + j 2 k ) ( 1 − cos 2 j x ) and hence ∫ 0 ∞ sin 2 k x 1 d x = ∫ 0 ∞ x 2 sin 2 k x d x = 2 2 k − 1 1 j = 1 ∑ k ( − 1 ) j − 1 ( k + j 2 k ) ∫ 0 ∞ x 2 1 − cos 2 j x d x = 2 2 k − 1 π j = 1 ∑ k ( − 1 ) j − 1 j ( k + j 2 k ) = 2 2 k − 1 π ( k − 1 2 k − 2 ) and hence k = 1 ∑ N + 1 ∫ 0 ∞ sin 2 k x 1 d x = π k = 0 ∑ N 2 2 k + 1 1 ( k 2 k ) = 2 N + 1 N ! π ( 2 N + 1 ) ! ! = Γ ( N + 1 ) π Γ ( N + 2 3 ) Putting N = 2 0 1 7 gives the answer Γ ( 2 0 1 8 ) π Γ ( 2 4 0 3 7 ) and hence the answer is 1 + 4 0 3 7 + 2 + 2 0 1 8 = 6 0 5 8
Alternatively, we can match the original integrals with the more standard Wallis integrals as follows: ∫ 0 ∞ sin 2 k x 1 d x = ∫ 0 ∞ x 2 sin 2 k x d x = 2 1 ∫ − ∞ ∞ x 2 sin 2 k x d x = 2 1 n ∈ Z ∑ ∫ 0 π ( x + n π ) 2 sin 2 k x d x = 2 1 ∫ 0 π sin 2 k x c o s e c 2 x d x = 2 1 ∫ 0 π sin 2 k − 2 x d x = ∫ 0 2 1 π sin 2 k − 2 x d x = 2 2 k − 1 π ( k − 1 2 k − 2 )