A q double product integral

Calculus Level 5

0 1 k = 0 ( 1 q 4 ( k + 1 ) ) ( 1 + q 2 k ) d q \int _{ 0 }^{ 1 }{ \prod _{ k=0 }^{ \infty }{ \left( 1{ -q }^{ 4(k+1) } \right) \left( 1+{ q }^{ 2k } \right) } } dq Express your solution up to 3 decimal places.


The answer is 3.5962945611.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Syed Shahabudeen
Jan 25, 2019

Although my solution is same as that of sir Mark Hennings , for the sake of remembrance of Ramanujan I'll make use of his theta function. we know that the Ramanujan theta function is defined as f ( a , b ) = n = a n ( n + 1 ) 2 b n ( n 1 ) 2 = ( a ; a b ) ( b ; a b ) ( a b ; a b ) f\left( a,b \right) =\sum _{ n=-\infty }^{ \infty }{ { a }^{ \frac { n(n+1) }{ 2 } }{ b }^{ \frac { n(n-1) }{ 2 } }={ \left( -a;ab \right) }_{ \infty }{ \left( -b;ab \right) }_{ \infty }{ \left( ab;ab \right) }_{ \infty } } when a = q 2 a={ q }^{ 2 } and b = 1 b=1 we have f ( q 2 , 1 ) = n = q n 2 + n = ( q 2 ; q 2 ) ( 1 ; q 2 ) ( q 2 ; q 2 ) f\left( { q }^{ 2 },1 \right) =\sum _{ n=-\infty }^{ \infty }{ { q }^{ { n }^{ 2 }+n }={ \left( -{ q }^{ 2 };{ q }^{ 2 } \right) }_{ \infty }{ \left( -1;{ q }^{ 2 } \right) }_{ \infty }{ \left( { q }^{ 2 };{ q }^{ 2 } \right) }_{ \infty } } where ( a ; q ) = k = 0 ( 1 a q k ) { \left( a;q \right) }_{ \infty }=\prod _{ k=0 }^{ \infty }{ \left( 1-{ aq }^{ k } \right) } is a pochammer notation. we came to see that the following product equates to an infinite bilateral series.Therefore we have the result as 0 1 n = q n 2 + n d q = n = 1 n 2 + n + 1 = π i 3 ( cot ( π ( 1 + i 3 2 ) ) cot ( π ( 1 i 3 2 ) ) ) = 3.5962945.. \int _{ 0 }^{ 1 }{ \sum _{ n=-\infty }^{ \infty }{ q^{ { n }^{ 2 }+n }dq=\sum _{ n=-\infty }^{ \infty }{ \frac { 1 }{ { n }^{ 2 }+n+1 } } =\frac { -\pi }{ i\sqrt { 3 } } } \left( \cot\left( \pi \left( \frac { 1+i\sqrt { 3 } }{ 2 } \right) \right) -\cot\left( \pi \left( \frac { 1-i\sqrt { 3 } }{ 2 } \right) \right) \right) =3.5962945.. }

Mark Hennings
Jan 25, 2019

By the Jacobi Triple Product formula, we have k = 0 ( 1 q 4 ( k + 1 ) ) ( 1 + q 2 k ) = k = 1 ( 1 q 4 k ) ( 1 + q 2 k 2 ) = k = 1 ( 1 q 2 k ) ( 1 + q 2 k 2 ) ( 1 + q 2 k ) = m Z q m 2 + m \begin{aligned} \prod_{k=0}^\infty \big(1 - q^{4(k+1)}\big)\big(1 + q^{2k}\big) & = \; \prod_{k=1}^\infty \big(1 - q^{4k}\big)\big(1 + q^{2k-2}\big) \\ &= \; \prod_{k=1}^\infty \big(1 - q^{2k}\big)\big(1 + q^{2k-2}\big)\big(1 + q^{2k}\big) \; = \; \sum_{m \in \mathbb{Z}} q^{m^2 + m} \end{aligned} for 0 < q < 1 0 < q < 1 , and hence 0 1 k = 0 ( 1 q 4 ( k + 1 ) ) ( 1 + q 2 k ) d q = m Z 1 m 2 + m + 1 = 2 π 3 tanh ( π 3 2 ) \int_0^1 \prod_{k=0}^\infty \big(1 - q^{4(k+1)}\big)\big(1 + q^{2k}\big)\,dq \; = \; \sum_{m \in \mathbb{Z}} \frac{1}{m^2 + m +1} \; = \; \frac{2\pi}{\sqrt{3}}\tanh\left(\frac{\pi\sqrt{3}}{2}\right) making the answer 3.596294561 \boxed{3.596294561} .

@Mark Hennings Sir, the last sum was evaluated by me using the Series representation of the Digamma Function..........Is there any other method???

Aaghaz Mahajan - 2 years, 4 months ago

Log in to reply

Integrate cot π z z 2 + z + 1 \frac{\cot \pi z}{z^2 + z + 1} around the square contour S n S_n with vertices ( n + 1 2 ) ± ( n + 1 2 ) i (n+\tfrac12) \pm(n+\tfrac12)i and ( n + 1 2 ) ± ( n + 1 2 ) i -(n+\tfrac12) \pm(n+\tfrac12)i . There exists K > 0 K > 0 such that cot π z K |\cot \pi z| \le K for all z S n z \in S_n for all n N n \in \mathbb{N} , so the value of the sum now comes from Cauchy's Residue Theorem and taking a limit: m = 1 m 2 + m + 1 + π R e s z = ω cot π z z 2 + z + 1 + π R e s z = ω 2 cot π z z 2 + z + 1 = 0 \sum_{m=-\infty}^\infty \frac{1}{m^2+m+1} + \pi\mathrm{Res}_{z = \omega}\frac{\cot \pi z}{z^2 + z + 1} + \pi\mathrm{Res}_{z = \omega^2}\frac{\cot \pi z}{z^2 + z + 1} \; = \; 0 where ω \omega is a primitive cube root of unity.

Mark Hennings - 2 years, 4 months ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...