Hot Integral - 8

Calculus Level 5

0 x 6 ( 10 x 9 + 8 ) 7 d x \displaystyle \int\limits_0^{\infty}\frac{x^6}{(10x^9 + 8)^7} \, dx

Given the above integral can be expressed as Γ ( A B ) Γ ( C B ) D E F G H I G \displaystyle \frac{\Gamma(\frac{A}{B})\Gamma(\frac{C}{B})}{DE^{\frac{F}{G}}H^{\frac{I}{G}}} where A , B , C , D , E , F , G , H , I A,B,C,D,E,F,G,H ,I all are positive integers.

Find the value of A + B + C + D + E + F + G + H + I A+B+C+D+E+F+G+H+I .

Details and Assumptions:

1) gcd ( A , B ) = gcd ( C , B ) = gcd ( F , G ) = gcd ( I , G ) = 1 \gcd(A,B)=\gcd(C,B)=\gcd(F,G)=\gcd(I,G)=1

2) Here Gamma functions may or may not be reducable. For more clarity 0 < A B < 1 0 < \frac{A}{B} < 1 and C B > 1 \frac{C}{B} > 1

3) Also F G < 1 \frac{F}{G}<1 and I G < 1 \frac{I}{G} <1

4) E E and H H are prime numbers.

Inspiration - Caboodle of Tricks .

This is a part of Hot Integrals


The answer is 3397386339.

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1 solution

Kartik Sharma
Aug 10, 2015

I will prove a general integral -

0 x a ( b + c x d ) e d x = Γ ( a + 1 d ) Γ ( e a + 1 d ) Γ ( e ) d b e a + 1 d c a + 1 d \displaystyle \int_{0}^{\infty}{\frac{{x}^{a}}{{\left(b + c{x}^{d}\right)}^{e}} dx} = \displaystyle \frac{\Gamma\left(\frac{a+1}{d}\right)\Gamma\left(e-\frac{a+1}{d}\right)}{\Gamma(e)d {b}^{e-\frac{a+1}{d}}{c}^{\frac{a+1}{d}}}

I will do this using a more non-conventional way using Ramanujan Master Theorem .

Substituting u = c b x d u = \frac{c}{b}{x}^{d} , d x = b c d x d 1 d u dx = \frac{b}{cd{x}^{d-1}}du

b c d b e 0 x a d + 1 d u ( 1 + u ) e \displaystyle \frac{b}{cd{b}^{e}}\int_{0}^{\infty}{\frac{{x}^{a-d+1} du}{{(1+u)}^{e}}}

1 c d b e 1 0 ( b u c ) a d + 1 d d u ( 1 + u ) e \displaystyle \frac{1}{cd{b}^{e-1}}\int_{0}^{\infty}{\frac{{\left(\frac{bu}{c}\right)}^{\frac{a-d+1}{d}} du}{{(1+u)}^{e}}}

1 d c a + 1 d b e a + 1 d 0 u a + 1 d 1 ( 1 + u ) e \displaystyle \frac{1}{d {c}^{\frac{a+1}{d}} {b}^{e-\frac{a+1}{d}}} \displaystyle \int_{0}^{\infty}{\displaystyle \frac{{u}^{\frac{a+1}{d}-1}}{{(1+u)}^{e}}}

Now the last integral is in the form of Mellin transform, so, we will use Ramanujan Master Theorem. We know that

1 ( 1 + x ) n = k = 0 Γ ( n + k ) Γ ( n ) ( x ) k k ! \displaystyle \frac{1}{{(1+x)}^{n}} = \sum_{k=0}^{\infty}{\frac{\Gamma(n+k)}{\Gamma(n)} \frac{{(-x)}^{k}}{k!}}

We get that our last integral will be equal to -

Γ ( a + 1 d ) Γ ( e a + 1 d ) \displaystyle \Gamma\left(\frac{a+1}{d}\right)\Gamma\left(e - \frac{a+1}{d}\right)

Hence our total integral becomes -

Γ ( a + 1 d ) Γ ( e a + 1 d ) Γ ( e ) d b e a + 1 d c a + 1 d \displaystyle\frac{\Gamma\left(\frac{a+1}{d}\right)\Gamma\left(e-\frac{a+1}{d}\right)}{\Gamma(e)d {b}^{e-\frac{a+1}{d}}{c}^{\frac{a+1}{d}}}

QED

Actually, the most difficult part of the problem is not what I have done. It's the calculations which follow.

@Aman Raimann Kindly change that d d in your problem to some more variable like what I have given. That d d makes the problem relatively boring. An answer like 3397386339 3397386339 is just not interesting, make it smaller. Please edit.

Kartik Sharma - 5 years, 10 months ago

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yaaa. but i cant chnge the answer now...

Aman Rajput - 5 years, 10 months ago

what is gamma function

Akshay Sharma - 5 years, 5 months ago

Y did u post the solution! (just kidding) I'm a newbie in RMT and even I did the same way.

Aditya Kumar - 5 years, 4 months ago

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