Limit with Integration - 6

Calculus Level 4

lim n 0 sin ( x n ) x n d x \displaystyle \large \lim_{n\to\infty}\int_0^\infty\frac{\sin(x^n)}{x^n}dx

Does not Exist 1 1 -1 -\infty π 2 \frac{\pi}{2} \infty 0 1 2 \frac12

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1 solution

Aman Rajput
Jun 4, 2021

sin ( x n ) 1 sin ( x n ) x n 1 x n |\sin(x^n)|\le1\therefore \frac{|\sin(x^n)|}{x^n}\le\frac{1}{x^n} 1 sin ( x n ) x n d x 1 x n d x = [ x 1 n n 1 ] 1 = 1 n 1 { n > 1 } \int_1^\infty\frac{\sin(x^n)}{x^n}dx\le\int_1^\infty x^{-n}dx=\left[\frac{x^{1-n}}{n-1}\right]_\infty^1=\frac{1}{n-1}\{n>1\} which converges to zero, now let us focus on the following portion of domain I = 0 1 sin ( x n ) x n d x I=\int_0^1\frac{\sin(x^n)}{x^n}dx we know that: lim x 0 sin ( x n ) x n 1 \lim_{x\to 0}\frac{\sin(x^n)}{x^n}\to1 and that is the region x [ 0 , 1 ] , sin ( x n ) x n 1 x\in[0,1],\,\,\frac{\sin(x^n)}{x^n}\le1 furthermore we can show that for n , sin ( x n ) x n 1 n\to\infty,\frac{\sin(x^n)}{x^n}\to1 and so taking the limit first we get: lim n 0 1 sin ( x n ) x n d x 0 1 d x = 1 \lim_{n\to\infty}\int_0^1\frac{\sin(x^n)}{x^n}dx\to\int_0^1dx=1 combining this with the first integral calculated and taking the limit we get our desired result.

Nice work, we can advanced this problem which yields us the unexpected value γ \gamma and ζ ( 2 ) \zeta(2) ( by bringing small changes in the problem).

Naren Bhandari - 1 week ago

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Jeff Giff - 6 days, 9 hours ago

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