Sunday, 23 August 2015

integration - Limit of an integral



I am trying to compute the following limit,
limk10|sin(kx)x|dx


After some numerical calculations, it loos like the limit is . To prove it, I tried to use Riemann sums but such approach is not working.



Any hints on how one should prove this ?




Thanks in advance.


Answer



Changing variables (y=kx) this is
limkk0|siny|ydy=0|siny|ydy.


This improper integral is well-known to diverge. For instance
nπ(n1)π|siny|ydy1nππ0sinydy=2nπ.

Adding these up, and considering the behaviour of the harmonic

series shows the integral diverges.


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