Monday, 1 December 2014

real analysis - Does the improper integral intlimits+infty0xpsinx,mathrmdx,  p>0 converge?




Does the improper integral +0xpsinxdx for p>0 converge?




Attempt. Limit of xpsinx as x tends to + does not exist, in order to guarantee integral's divergence. So I worked on the definition. For p=1 we get the integral:
+0xsinxdx=lim (after integration by parts), which does not converge (the limit does not exist). This approach also works for p>1, but I had difficulties regarding the case p<1.




Thank you in advance.


Answer



If that integral converged, then as n\to \infty



\int_{2\pi n}^{2\pi n + \pi} x^p\sin x\, dx\to 0.



But this integral is greater than



(2\pi n)^p\int_{2\pi n}^{2\pi n + \pi} \sin x\, dx = (2\pi n)^p\cdot 2 \to \infty, contradiction.



No comments:

Post a Comment

real analysis - How to find lim_{hrightarrow 0}frac{sin(ha)}{h}

How to find \lim_{h\rightarrow 0}\frac{\sin(ha)}{h} without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...