Resembling Resemblence-4

Calculus Level 5

k = 1 ( 1 ) k + 1 H k k = 1 2 ( ζ ( P ) log 2 ( Q ) ) \begin{aligned}\displaystyle\sum_{k=1}^{\infty} \dfrac{(-1)^{k+1} H_{k}}{k}\end{aligned}=\dfrac{1}{2} \left(\zeta(P)-\log^2 (Q)\right)

If the above equation is satisfied by positive integers P P and Q Q . Find P Q P^Q .


H n H_{n} is the n t h n^{th} harmonic number.


The answer is 4.

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1 solution

Parth Lohomi
Aug 26, 2015

Using integral representation n = 1 ( 1 ) n + 1 n H n = 0 1 n = 1 ( x ) n H n d x x \displaystyle\sum_{n=1}^\infty \dfrac{(-1)^{n+1}}{n} H_n = -\displaystyle\int_0^1 \sum_{n=1}^\infty (-x)^n H_n \dfrac{\mathrm{d} x }{x}

Now:

n = 1 ( x ) n H n = n = 1 x n k = 0 n 1 ( 1 ) k ( 1 ) n k n k -\displaystyle\sum_{n=1}^\infty (-x)^n H_n = -\displaystyle\sum_{n=1}^\infty x^n \displaystyle\sum_{k=0}^{n-1} (-1)^k \frac{(-1)^{n-k}}{n-k} = n = 0 ( x ) n k = 1 ( x ) k k = log ( 1 + x ) 1 + x = -\displaystyle\sum_{n=0}^\infty (-x)^n \cdot \displaystyle\sum_{k=1}^\infty \frac{(-x)^k}{k} = \frac{\log(1+x)}{1+x}

Thus

I = 0 1 log ( 1 + x ) 1 + x d x x = ( 1 2 log 2 ( 1 + x ) Li 2 ( x ) ) x = 0 x = 1 = 1 2 log 2 ( 2 ) Li 2 ( 1 ) I=\color{#3D99F6}{\displaystyle\int_0^1 \dfrac{\log(1+x)}{1+x} \dfrac{\mathrm{d}x}{x} = \left. \left(-\dfrac{1}{2} \log^2(1+x) - \text{Li}_2(-x) \right)\right|_{x = 0}^{x=1} = -\dfrac{1}{2} \log^2(2) - \text{Li}_2(-1)}

But Li 2 ( 1 ) = k = 1 ( 1 ) k k 2 = ( 2 1 2 1 ) ζ ( 2 ) = 1 2 ζ ( 2 ) \text{Li}_2(-1) = \sum_{k=1}^\infty \frac{(-1)^k}{k^2} = \left(2^{1-2}-1\right) \zeta(2) = -\frac{1}{2} \zeta(2) .Thus

I = 1 2 ( ζ ( 2 ) log 2 ( 2 ) ) \color{#D61F06}{\boxed{I=\dfrac{1}{2} \left( \zeta(2) - \log^2(2)\right)}}

Yeah first time your good problem took me less than 10 mins to solve. Even I did it in the same method, except for a few more steps. Your questions have strengthened me man. Thanks a lot! Do try my problem once. This one is inspired by u.

Aditya Kumar - 5 years, 9 months ago

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Thanks,and congratulations!

Parth Lohomi - 5 years, 9 months ago

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Welcome! U go to resonance right??

Aditya Kumar - 5 years, 9 months ago

hahahah gud one aditya :)

Aman Rajput - 5 years, 9 months ago

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@Aman Raimann your hot integrals r damn amazing. My next aim is to learn about the functions you've used and to solve your problem.

Aditya Kumar - 5 years, 9 months ago

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@Aditya Kumar haahaha ..thnxx dude.. try them !! i think they are much harder only 2 questions are easy ... :)

Aman Rajput - 5 years, 9 months ago

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@Aman Rajput Currently u r in which college??

Aditya Kumar - 5 years, 9 months ago

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@Aditya Kumar jamia millia islamia... see my profile

Aman Rajput - 5 years, 9 months ago

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@Aman Rajput I'm sorry but I haven't heard of this college. U didn't get into IIT??

Aditya Kumar - 5 years, 9 months ago

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@Aditya Kumar this is central govt university at delhi.. . i didnt get iit college because i am weak in chemistry :P

Aman Rajput - 5 years, 9 months ago

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@Aman Rajput This is the same problem I'm facing. Maths and physics r easy but chem :'(

Aditya Kumar - 5 years, 9 months ago

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@Aditya Kumar yaa right /// that tym i was just a zero in chemistry ...

well tomorrow is my interview in college so . gudnyt ... and hey my actual name is Aman Rajput.

i already mailed brilliant@support... in order to change my profile name... but got no response .. :'( so gudnyt buddyy .... we'll talk later... in another solved problem :D

Aman Rajput - 5 years, 9 months ago

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@Aman Rajput Haha goodnight Mr Rajput

Aditya Kumar - 5 years, 9 months ago

@Aditya Kumar you should study different functions.. in order to solve more problems by simpler methods .

for e.g. 0 π / 2 log ( \sinx ) d x \int\limits_0^{\pi/2} \log(\sinx) dx .this solved question in many textbooks of india with the famous procedure which u know very well... but if you know the polygamma you can solve it in 2 lines... instead of using 15 lines as in NCERT books..

Aman Rajput - 5 years, 9 months ago

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@Aman Rajput How much time would it take me to learn Polygamma function

Aditya Kumar - 5 years, 9 months ago

( link try to do this problem please

Sudhamsh Suraj - 4 years, 3 months ago

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