That's too much!

Calculus Level 5

Find the value of:

k = 1 1 k ( 1 + 12 k ) \sum _{ k=1 }^{ \infty }{ \frac { 1 }{ k\left( 1+12k \right) } }

The answer is of the form:

A B ( l n 2 + l n 3 C ) π ( D + E F ) + G H l n ( I J ) A-B\left( ln2+\frac { ln3 }{ C } \right) -\pi \left( D+\frac { \sqrt { E } }{ F } \right) +G\sqrt { H } ln\left( \sqrt { I-\sqrt { J } } \right)

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

Try more here!


The answer is 33.

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2 solutions

Aditya Kumar
Aug 25, 2015

k = 1 1 k ( 1 + 12 k ) = 1 12 k = 1 12 k ( 1 + 12 k ) = 1 12 k = 1 1 k ( 1 12 + k ) = H 1 12 = 12 3 ( l n 2 + l n 3 2 ) π ( 1 + 3 2 ) + 2 3 l n ( 2 3 ) \sum _{ k=1 }^{ \infty }{ \frac { 1 }{ k\left( 1+12k \right) } } \\ =\quad \frac { 1 }{ 12 } \sum _{ k=1 }^{ \infty }{ \frac { 12 }{ k\left( 1+12k \right) } } \\ =\quad \frac { 1 }{ 12 } \sum _{ k=1 }^{ \infty }{ \frac { 1 }{ k\left( \frac { 1 }{ 12 } +k \right) } } ={ H }_{ \frac { 1 }{ 12 } }\\ =12-3\left( ln2+\frac { ln3 }{ 2 } \right) -\pi \left( 1+\frac { \sqrt { 3 } }{ 2 } \right) +2\sqrt { 3 } ln\left( \sqrt { 2-\sqrt { 3 } } \right)

How did you jump from line 3 to line 4?

Or how do you calculate H(1/12)?

Pi Han Goh - 5 years, 9 months ago

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Actually I used the value of H 12 {H}_{12} given in Wikipedia. There was a lengthy method to calculate it's value.

Aditya Kumar - 5 years, 9 months ago

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That's the point. It's not easy to evaluate stuff like these. It's better to post this as a note and ask the readers to prove it.

Pi Han Goh - 5 years, 9 months ago

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@Pi Han Goh I'm trying to evaluate it's value. Once I get with it I'll surely post it.

Aditya Kumar - 5 years, 9 months ago

@Pi Han Goh I'll post the value of H1/12 tonight.

Aditya Kumar - 5 years, 9 months ago

@Aditya Kumar Have you seen the generalization I have provided to do such sums? If not, you should give it a look at. How Wikipedia calculated H 1 / 12 {H}_{1/12} can be explained by using the definition of digamma function and using the same(polygamma function), you can prove a general identity which I have provided in one of the Pi Han Goh's problem. There, I've not provided with the proof though but the generalization is really helpful to remember.

Kartik Sharma - 5 years, 9 months ago

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Finally found the link .

Kartik Sharma - 5 years, 9 months ago

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Yeah, I'll look up to it

Aditya Kumar - 5 years, 9 months ago

Yes I actually had forgotten to prove that. I'll prove that sometime later as my exams r on. Till then try my new problem. I'm sure you'll solve it.

Aditya Kumar - 5 years, 9 months ago

@Abhishek Bakshi see this and comment.

Aditya Kumar - 5 years, 8 months ago

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You are actually giving free points to people... and why are you wasting your time on the digamma and polygamma functions functions.. it would be better if you prepared more seriously about IIT or for some reputed mathematics institution.. you can do all this after you get into them..

Abhishek Bakshi - 5 years, 7 months ago

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I am preparing for IIT. It is just that I give 1hr daily to my interests.

Aditya Kumar - 5 years, 7 months ago

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@Aditya Kumar great, continue... giving free points :-D

Abhishek Bakshi - 5 years, 7 months ago

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@Abhishek Bakshi Y not solve all? I see u have solved only 2 of my probs

Aditya Kumar - 5 years, 7 months ago

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@Aditya Kumar don't have so much free time as u..

Abhishek Bakshi - 5 years, 7 months ago

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@Abhishek Bakshi Good joke! Just accept u couldn't solve all. U solved only those which use digamma. Try other probs.

Aditya Kumar - 5 years, 7 months ago

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@Aditya Kumar oh...really!!!! even if its true, it doesn't matter cause you are a flopshow in the academy tests.... and neither do i have the need to solve all of your unimportant problems..

Abhishek Bakshi - 5 years, 7 months ago
Aman Rajput
Sep 1, 2015

r = 1 n 1 r + a = ψ ( n + a + 1 ) ψ ( a + 1 ) \displaystyle\sum_{r=1}^{n} \frac{1}{r+a} = \psi(n+a+1)-\psi(a+1)

Split the given fraction as 1 k ( 1 + 12 k ) = 1 k 1 k + 1 12 \frac{1}{k(1+12k)} = \frac{1}{k} - \frac{1}{k+\frac{1}{12}}

Taking summations upto n n both the sides you get

k = 1 n 1 k 1 k + 1 12 = ψ ( n + 1 ) ψ ( 1 ) ψ ( n + 13 12 ) + ψ ( 13 12 ) \displaystyle \sum_{k=1}^{n} \frac{1}{k} - \frac{1}{k+\frac{1}{12}}= \psi(n+1) - \psi(1) - \psi(n + \frac{13}{12}) + \psi(\frac{13}{12})

using ψ ( 1 ) = γ \psi(1) = -\gamma

and

Take limits n n to \infty both the sides we get...

ψ ( 13 12 ) + γ \displaystyle\boxed{\psi(\frac{13}{12}) + \gamma}

I didn't get anything :P. Yor questions appear to me like this

Aditya Kumar - 5 years, 9 months ago

wolframalpha gives directly the partial sum formula for this...

but i didnt know the property u used

Ciara Sean - 5 years, 9 months ago

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He used polygamma function

Aditya Kumar - 5 years, 9 months ago

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here its digamma

Aman Rajput - 5 years, 9 months ago

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@Aman Rajput Yes. I'll start learning it after a few days.

Aditya Kumar - 5 years, 9 months ago

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