Resembling Resemblence-3

Calculus Level 5

0 log 1 + x 3 x 3 x d x 1 + x 3 = π M log M π N M 2 . \large\displaystyle\int_0^\infty \log \dfrac{1+x^3}{x^3} \dfrac{x \,dx}{1+x^3}=\dfrac{\pi}{\sqrt M}\log M-\dfrac{\pi^N}{M^2}.

The equation above is true for M M and N N are positive integers. Find M + N M+N .


The answer is 5.

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1 solution

Parth Lohomi
Aug 18, 2015

Define

I ( a ) = 0 log a + x 3 x 3 x d x 1 + x 3 \color{#D61F06}{I(a)=\displaystyle\int_0^\infty \log \dfrac{a+x^3}{x^3} \dfrac{x \,dx}{1+x^3}}

Then I ( 0 ) = 0 I(0)=0 and

I ( a ) = 0 x ( a + x 3 ) ( 1 + x 3 ) d x = 1 3 0 1 x 1 / 3 ( a + x ) ( 1 + x ) d x = 1 3 ( 1 a ) ( 0 1 x 1 / 3 ( a + x ) d x 0 1 x 1 / 3 ( 1 + x ) d x ) = 1 3 ( 1 a ) 2 π 3 ( 1 a 1 / 3 1 ) = 2 π 3 3 1 a + a 2 / 3 + a 1 / 3 \color{#3D99F6}{\begin{aligned} I'(a)&=&\displaystyle\int_0^\infty\dfrac{x}{(a+x^3)(1+x^3)}dx\\ &=&\dfrac13\displaystyle\int_0^\infty\dfrac{1}{x^{1/3}(a+x)(1+x)}dx\\ &=&\dfrac{1}{3(1-a)}\left(\displaystyle\int_0^\infty\dfrac{1}{x^{1/3}(a+x)}dx-\int_0^\infty\dfrac{1}{x^{1/3}(1+x)}dx\right)\\ &=&\dfrac{1}{3(1-a)}\dfrac{2\pi}{\sqrt3}\left(\frac{1}{a^{1/3}}-1\right)\\ &=&\dfrac{2\pi}{3\sqrt3}\dfrac{1}{a+a^{2/3}+a^{1/3}} \end{aligned}}

Here we use

0 1 x 1 / 3 ( a + x ) d x = 2 π 3 a 1 / 3 \color{#20A900}{\displaystyle\int_0^\infty\dfrac{1}{x^{1/3}(a+x)}dx=\dfrac{2\pi}{\sqrt3 a^{1/3}}}

Thus

I ( 1 ) = 2 π 3 0 1 1 a + a 2 / 3 + a 1 / 3 d a = 2 π 3 3 0 1 b b 2 + b + 1 d b = π 2 9 + π 3 ln 3 . \color{#3D99F6}{\begin{aligned} I(1)&=&\frac{2\pi}{\sqrt3}\int_0^1 \frac{1}{a+a^{2/3}+a^{1/3}}da\\ &=&\frac{2\pi}{3\sqrt3}\int_0^1 \frac{b}{b^2+b+1}db\\ &=&-\boxed{\dfrac{\pi^2}{9}+\dfrac{\pi}{\sqrt3}\ln 3}. \end{aligned}}

Nice method :)

Bonus: Can you generalize?

0 x a 1 + x n ln ( 1 + x n x m ) d x \displaystyle \int_0^{\infty} \frac{x^a}{1+x^n} \ln \big(\frac{1+x^n}{x^m} \big) dx

Hasan Kassim - 5 years, 9 months ago

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@hasan kassim Will you mind if I take this generalization to the Calculus Contest ?

But of course, then you will not be able to post the solution. But I have thought something so that you are not disadvantaged. Will you accept my proposal for being a staff member in this contest so that we both may post challenges(and then of course, when the contest ends, post the solutions(if required))?

Kartik Sharma - 5 years, 9 months ago

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Of course I accept! My pleasure :)

You can post this generalization if you want , It is all yours!

Hasan Kassim - 5 years, 9 months ago

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@Hasan Kassim First of all thanks! I will try to wait.

Next of all, sorry I can't wait. This is the generalization-

π n c s c ( π ( a + 1 ) n ) ( m π n c o t ( π ( a + 1 ) n ) ( ψ ( n a 1 n ) + γ ) ) \displaystyle \frac{\pi}{n}csc\left(\frac{\pi(a+1)}{n}\right)\left(\frac{m\pi}{n} cot\left(\frac{\pi(a+1)}{n}\right) - \left(\psi\left(\frac{n-a-1}{n}\right) +\gamma\right)\right)

Isn't it?

Kartik Sharma - 5 years, 9 months ago

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@Kartik Sharma Nope

you should take out the 1/n behind the digamma

Hasan Kassim - 5 years, 9 months ago

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@Hasan Kassim Yeah sorry. A big typo! I forgot that I have already taken that 1/n out. Now, is it fine?

Kartik Sharma - 5 years, 9 months ago

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@Kartik Sharma yes exactly !

Btw, did you solve it using complex??

Hasan Kassim - 5 years, 9 months ago

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@Hasan Kassim No. As usual I have used Ramanujan Master Theorem which is somehow a consequence of complex. I am yet to take a full course in complex. Just facing a time constraint! I know basic to intermediate level complex though. BTW, where did you learn complex?

Kartik Sharma - 5 years, 9 months ago

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@Kartik Sharma Actually I don't have any idea about complex analysis! That's why , as you can see, all my solutions are elementary and don't have any relation to complex... I have just learned the basics of complex; you know, the easy algebra topics. That's my domain in complex so far!!

I am trying to learn , but don't have a good book for complex :\

Hasan Kassim - 5 years, 9 months ago

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@Hasan Kassim Try this book: Complex Variables and Applications by Ruel .V. Churchill, James W. Brown and Roger F. Verhey. It's good.

Kishore S. Shenoy - 5 years, 9 months ago

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@Kishore S. Shenoy Thanks Kishore!

Can you provide me a direct link?

Hasan Kassim - 5 years, 9 months ago

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@Hasan Kassim Link of what ¨ \ddot \smile ?

Kishore S. Shenoy - 5 years, 9 months ago

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@Kishore S. Shenoy A link to reach the book or download link, isn't the book on the internet??

Hasan Kassim - 5 years, 9 months ago

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@Hasan Kassim http://bookfi.org/dl/1374590/ed64f7

Kishore S. Shenoy - 5 years, 9 months ago

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@Kishore S. Shenoy thanks a ton!!

Hasan Kassim - 5 years, 9 months ago

@Hasan Kassim I think you should check out Advanced Calculus - Wilfred Kaplan. It is not specifically a book for complex analysis(though it has a chapter named so) but it's a great book, seriously. Vectors, calculus, integration(line, surface etc.) etc. everything you would get interested in is in that book.

Kartik Sharma - 5 years, 9 months ago

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@Kartik Sharma Thank you Kartik :)

Hasan Kassim - 5 years, 9 months ago

or if you can give me some time, tomorrow I will post the note , and you attach it to your set.

Hasan Kassim - 5 years, 9 months ago

awsome method. only intuition would help! +1

Aditya Kumar - 5 years, 9 months ago

You are 14, and how are you going to write JEE with us in 2017?

Kishore S. Shenoy - 5 years, 9 months ago

i thought about using this method. can we solve it using infinite series.

Srikanth Tupurani - 1 year, 9 months ago

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