Tribute to Ramanujan on his upcoming birthday!

Calculus Level 5

0 [ ( 1729 x + 11957764 + x 1729 ( x + 1729 ) + 11957764 + ( x + 1729 ) 3458 ) 1 2 ( γ x + l n Γ ( 1 + x ) ) x 2 ] d x \large \large \large \large \large \large \large \large \large \large \int _{ 0 }^{ \infty }{ \left[ \frac { { \left( \sqrt { 1729x+11957764+x\sqrt { 1729\left( x+1729 \right) +11957764+\left( x+1729 \right) \sqrt { \ldots } } } -3458 \right) }^{ \frac { -1 }{ 2 } }\left( \gamma x+ln\Gamma \left( 1+x \right) \right) }{ { x }^{ 2 } } \right] } dx

Evaluate the integral above. Take ζ ( 3 2 ) = 2.612 \zeta \left( \frac { 3 }{ 2 } \right) =2.612 .

Extra Credits:

  • Find a general solution to the problem.

  • The first person who posts the solution explaining all the steps wins my respect.


The answer is 5.47.

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3 solutions

Tanishq Varshney
Dec 17, 2015

As the question is a tribute to Sir Ramanujan, the question involves two things related to him.

1 ) N e s t e d R a d i c a l 1) ~ Nested ~ Radical equation 26

2 ) Ramanujan Master Theorem 2)~\text{Ramanujan Master Theorem}


x + n + a = a x + ( n + a ) 2 + x a ( x + n ) + ( n + a ) 2 + . . . . . \large{x+n+a=\sqrt{ax+(n+a)^2+x \sqrt{a(x+n)+(n+a)^2+.....}}}

It would be nice if someone provides the proof for this because i don't know :P

Here n = a = 1729 n=a=1729

Also

ln ( Γ ( 1 + x ) ) = γ x + k = 2 ζ ( k ) ( k ) ( x ) k \large{\ln(\Gamma (1+x))=- \gamma x+ \displaystyle \sum_{k=2}^{\infty}\frac{\zeta(k)}{(k)}(-x)^{k}}

ln ( Γ ( 1 + x ) ) = γ x + k = 0 ζ ( k + 2 ) ( k + 2 ) ( x ) k + 2 \large{\ln(\Gamma (1+x))=- \gamma x+ \displaystyle \sum_{k=0}^{\infty}\frac{\zeta(k+2)}{(k+2)}(-x)^{k+2}}

For Proof see Problem number 7 at the last.

ln ( Γ ( 1 + x ) ) + γ x x 2 = k = 0 Γ ( k + 1 ) ζ ( k + 2 ) ( k + 2 ) ( x ) k k ! \large{ \rightarrow \frac{\ln(\Gamma (1+x))+\gamma x}{x^2}= \displaystyle \sum_{k=0}^{\infty}\frac{\Gamma(k+1) \zeta(k+2)}{(k+2)}\frac{(-x)^{k}}{k!}}

Thus the integral now becomes

0 ( x + 1729 + 1729 3458 ) 1 2 ln ( Γ ( 1 + x ) ) + γ x x 2 d x \large{\displaystyle \int^{\infty}_{0} (x+1729+1729-3458)^{- \frac{1}{2}} \frac{\ln(\Gamma (1+x))+\gamma x}{x^2} dx}

= 0 x 1 2 ln ( Γ ( 1 + x ) ) + γ x x 2 d x \large{= \displaystyle \int^{\infty}_{0} x^{- \frac{1}{2}} \frac{\ln(\Gamma (1+x))+\gamma x}{x^2} dx}

Now ϕ ( k ) = Γ ( k + 1 ) ζ ( k + 2 ) ( k + 2 ) \large{\phi(k)=\frac{\Gamma(k+1) \zeta(k+2)}{(k+2)}}

thus by Ramanujan Master theorem

= 0 x s 1 ln ( Γ ( 1 + x ) ) + γ x x 2 d x = Γ ( s ) ϕ ( s ) \large{= \displaystyle \int^{\infty}_{0} x^{s-1} \frac{\ln(\Gamma (1+x))+\gamma x}{x^2} dx=\Gamma(s) \phi(-s)}

= 0 x s 1 ln ( Γ ( 1 + x ) ) + γ x x 2 d x = Γ ( s ) Γ ( 1 s ) ζ ( 2 s ) 2 s \large{= \displaystyle \int^{\infty}_{0} x^{s-1} \frac{\ln(\Gamma (1+x))+\gamma x}{x^2} dx=\frac{\Gamma(s) \Gamma(1-s) \zeta(2-s)}{2-s}}

here s = 1 2 s=\frac{1}{2}

Thus we have by euler reflection formula

= π ζ ( 3 / 2 ) ( 2 1 2 ) sin ( π 2 ) \Large{=\frac{\pi \zeta(3/2)}{\left( 2-\frac{1}{2} \right) \sin \left(\frac{\pi}{2} \right)}}

2 π 3 ζ ( 3 / 2 ) = 5.46 \Large{\frac{2 \pi }{3} \zeta(3/2)=\boxed{5.46}}

Elegant solution :)

Swapnil Das - 5 years, 5 months ago

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I didn't even understand the solution!Swapnil!

Adarsh Kumar - 5 years, 5 months ago

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Which part u didn't understand?

Aditya Kumar - 5 years, 5 months ago

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@Aditya Kumar Actually I am not aware of Ramanujan Master Theorem and those integrals which involve gamma function.

Adarsh Kumar - 5 years, 5 months ago

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@Adarsh Kumar Hmm. Try learning them

Aditya Kumar - 5 years, 5 months ago

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@Aditya Kumar I will definitely try bhaiya!

Adarsh Kumar - 5 years, 5 months ago

You don't need to understand everything to feel the beauty of Mathematics.

Swapnil Das - 5 years, 5 months ago

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@Swapnil Das True,but to understand this particular solution i need to.:P

Adarsh Kumar - 5 years, 5 months ago

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@Adarsh Kumar Then read up the part you didn't understand.

Swapnil Das - 5 years, 5 months ago

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@Swapnil Das Actually then Swapnil I would have to start and stop at the beginning :P!lolol

Adarsh Kumar - 5 years, 5 months ago

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@Adarsh Kumar Do you know limits and derivatives?

Swapnil Das - 5 years, 5 months ago

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@Swapnil Das Yes,but i don't know gamma function and zeta function.

Adarsh Kumar - 5 years, 5 months ago

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@Adarsh Kumar Then, why don't you read integration ?

Swapnil Das - 5 years, 5 months ago

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@Swapnil Das That also involves gamma function,sorry.

Adarsh Kumar - 5 years, 5 months ago

@Swapnil Das SD u type really well.

Aditya Kumar - 5 years, 5 months ago

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@Aditya Kumar Erm.. Thanks. Did you appreciate my typing speed, my way of typing letters or the style of my writing?

Swapnil Das - 5 years, 5 months ago

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@Swapnil Das U write really well. Your dialogues are really awesome :P

Aditya Kumar - 5 years, 5 months ago

I know the proof of Ramanujan Nested theorem

Aman Rajput - 5 years, 5 months ago

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Yeah even I know that:). How was the problem? Did u use rmt?

Aditya Kumar - 5 years, 5 months ago

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Ya rmt .. Good problem

Aman Rajput - 5 years, 5 months ago

It also involves his famous taxicab number 1729 :) Nice solution.

Kitran Colwell - 5 years, 4 months ago

Try to prove the radical then you'll win my respect.

Aditya Kumar - 5 years, 5 months ago

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I read many PDFs for its proof , all of them stated to consider some function and then it was easy to go after it.

Tanishq Varshney - 5 years, 5 months ago

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Yes u r right.

Aditya Kumar - 5 years, 5 months ago

Hi Tanishq are you going on 15th or 16th

Department 8 - 5 years, 5 months ago

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@Department 8 what do u mean by that , sorry i didn't understand?

Tanishq Varshney - 5 years, 5 months ago

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@Tanishq Varshney WWE Live India

Department 8 - 5 years, 5 months ago

I know the proof.. If you too know ?? Then I don't need to post that :)

Aman Rajput - 5 years, 5 months ago
Aman Rajput
Dec 22, 2015

I solved it. @Aditya Kumar What do you want to generalise .. What tanishq gave ..are you satisfied with that ?

Ronak Agarwal
Dec 16, 2015

The answer is = 2 3 π ζ ( 3 2 ) = \dfrac{2}{3}\pi \zeta{(\dfrac{3}{2})}

U didn't win my respect :P. Please post the complete solution.

Aditya Kumar - 5 years, 6 months ago

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