Prime sine power

Calculus Level 4

v ( a ) = i = 1 a 0 π sin P i ( x P i ) d x v(a) = \sum_{i=1}^a \int_0^\pi \sin^{P_i}\left(x P_i\right) \text{d}x

Consider the sum of integrals above where P i P_i is the i i -th prime number ( P 1 = 2 P_1 = 2 ).

v v is of the form p q + π 2 \dfrac{p}{q} + \dfrac{\pi}{2} . Give q p q-p for v ( 6 ) v(6) .


The answer is 2304213.

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1 solution

For P 1 = 2 P_1 = 2 , I 2 = 0 π sin 2 ( 2 x ) d x = 0 π 1 2 cos ( 4 x ) 2 d x = x 8 sin ( 4 x ) 2 0 π = π 2 \displaystyle I_2 = \int_0^\pi \sin^2 (2x) \ dx = \int_0^\pi \frac {1-2\cos (4x)}2 \ dx = \frac {x-8\sin (4x)}2 \bigg|_0^\pi = \frac \pi 2

Besides P 1 = 2 P_1 = 2 , the other primes are odd. Then, we have:

I k = 0 π sin k ( k x ) d x Since the integrand is odd = 0 π k sin k ( k x ) d x Let u = k x , d u = k d x = 1 k 0 π sin k u d u Since the integrand is symmetrical on π 2 = 2 k 0 π 2 sin k u cos 0 u d u = 1 k B ( k + 1 2 , 1 2 ) B ( m , n ) is beta function. = Γ ( k + 1 2 ) Γ ( 1 2 ) k Γ ( k + 2 2 ) Γ ( n ) is gamma function. = ( k 1 2 ) ! π k k ! ! 2 k + 1 2 π = 2 k + 1 2 ( k 1 2 ) ! k k ! ! \begin{aligned} I_k & = \int_0^\pi \sin^k (kx) \ dx & \small {\color{#3D99F6}\text{Since the integrand is odd}} \\ & = \int_0^\frac \pi k \sin^k (kx) \ dx & \small {\color{#3D99F6}\text{Let }u=kx, \ du = k \ dx} \\ & = \frac 1k \int_0^\pi \sin^k u \ du & \small {\color{#3D99F6}\text{Since the integrand is symmetrical on }\frac \pi 2} \\ & = \frac 2k \int_0^\frac \pi 2 \sin^k u \cos^0 u \ du \\ & = \frac 1k B \left(\frac {k+1}2, \frac 12 \right) & \small {\color{#3D99F6} B(m,n) \text{ is beta function.}} \\ & = \frac {\Gamma \left(\frac {k+1}2 \right) \Gamma \left( \frac 12 \right)}{k \Gamma \left(\frac {k+2}2 \right)} & \small {\color{#3D99F6} \Gamma (n) \text{ is gamma function.}} \\ & = \frac {\left(\frac {k-1}2 \right)! \sqrt \pi}{k \frac {k!!}{2^{\frac {k+1}2}} \sqrt \pi} \\ & = \frac {2^{\frac {k+1}2}\left(\frac {k-1}2 \right)!}{k \cdot k!!} \end{aligned}

Therefore, we have:

v ( 6 ) = π 2 + 2 2 1 ! 3 3 ! ! + 2 3 2 ! 5 5 ! ! + 2 4 3 ! 7 7 ! ! + 2 6 5 ! 11 11 ! ! + 2 7 6 ! 13 13 ! ! = π 2 + 4 9 + 16 75 + 32 245 + 512 7623 + 2048 39039 = 22745812 25050025 \begin{aligned} v(6) & = \frac \pi 2 + \frac {2^2 \cdot 1!}{3 \cdot 3!!} + \frac {2^3 \cdot 2!}{5 \cdot 5!!} + \frac {2^4 \cdot 3!}{7 \cdot 7!!} + \frac {2^6 \cdot 5!}{11 \cdot 11!!} + \frac {2^7 \cdot 6!}{13 \cdot 13!!} \\ & = \frac \pi 2 + \frac 49 + \frac {16}{75} + \frac {32}{245} + \frac {512}{7623}+ \frac {2048}{39039} \\ & = \frac {22745812}{25050025} \end{aligned}

q p = 25050025 22745812 = 2304213 \implies q-p = 25050025 - 22745812 = \boxed{2304213}

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