Monday, 1 December 2014

real analysis - Does the improper integral $intlimits_0^{+infty}x^psin x,mathrm{d}x,~~p>0$ converge?




Does the improper integral $\int\limits_0^{+\infty}x^p\sin x\,\mathrm{d}x$ for $p>0$ converge?




Attempt. Limit of $x^p\sin x$ as $x$ tends to $+\infty$ does not exist, in order to guarantee integral's divergence. So I worked on the definition. For $p=1$ we get the integral:
$$\int\limits_0^{+\infty}x\sin x\,\mathrm{d}x=\lim_{x\to +\infty}(\sin x-x\cos x)$$ (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.



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