Nice integral

Calculus Level 4

0 1 ln ( 1 + x 4 ) x d x = π A B \large{ \int_0^1 \dfrac{\ln(1+x^4)}{x}\ \mathrm{d}x = \dfrac{\pi^A}{B}}

If for positive integers A , B A,B , the above integral satisfies, submit the value of ϕ ( B A ) \phi(B-A) as your answer where ϕ ( s ) \phi(s) denotes the Euler-Totient Function.


The answer is 22.

This section requires Javascript.
You are seeing this because something didn't load right. We suggest you, (a) try refreshing the page, (b) enabling javascript if it is disabled on your browser and, finally, (c) loading the non-javascript version of this page . We're sorry about the hassle.

2 solutions

Kunal Gupta
Sep 23, 2015

Consider. I = 0 1 ln ( 1 + x p ) d x x I =\displaystyle \int_0^1 \dfrac{\ln(1+x^p)dx}{x} as, ln ( 1 + x ) = n = 1 x n ( 1 ) n + 1 n \ln(1+x) = \displaystyle \sum_{n=1} ^ {\infty} \dfrac{ x^n(-1)^{n+1}}{n} plugging x x p x \rightarrow x^p ln ( 1 + x p ) = n = 1 x n p ( 1 ) n + 1 n \ln(1+x^p) = \displaystyle \sum_{n=1} ^ {\infty} \dfrac{ x^np(-1)^{n+1}}{n} Note that as the integral has the limits from 0 0 to 1 1 so the above series is valid.
Hence I = 0 1 n = 1 x n p ( 1 ) n + 1 d x n x = n = 1 ( 1 ) n + 1 n 0 1 x n p 1 d x = n = 1 ( 1 ) n + 1 p n 2 I =\displaystyle \int_0 ^1 \sum_{n=1} ^ {\infty} \dfrac{ x^np (-1)^{n+1} dx}{nx} =\displaystyle \sum_{n=1} ^ {\infty} \dfrac{(-1)^{n+1}}{n} \int_{0} ^ {1} x^{np-1} dx = \displaystyle \sum_{n=1} ^ {\infty} \dfrac{(-1)^{n+1}}{pn^2 }
The second step is verified by Fubini-Tonelli's theorem
By the fact that n = 1 ( 1 ) n + 1 n 2 = π 2 12 \displaystyle \sum_{n=1} ^ {\infty} \dfrac{(-1)^{n+1}}{n^2} = \dfrac{\pi^2}{12} Hence, I = π 2 12 p I= \dfrac{\pi^2}{12p} For our case , plug p = 4 p=4 , so I = π 2 48 I= \dfrac{\pi^2}{48} Clearly, ϕ ( 48 2 ) = ϕ ( 46 ) = 22 \phi(48-2)=\phi(46) = \boxed{\boxed{22}} Q.E.D \large{ \text{Q.E.D} }


Use series expansion of l n ( 1 + x ) ln(1+x) yields the values as ζ ( 2 ) / 8 = I = π 2 / 48 \zeta (2)/8=I=π^2/48

Spandan Senapati - 4 years ago

0 pending reports

×

Problem Loading...

Note Loading...

Set Loading...