Saturday, May 13, 2017

Does the series $sum_{n=1}^infty frac{n}{sqrt[3]{8n^5-1}}$ Converge?


$$\sum_{n=1}^\infty \frac{n}{\sqrt[3]{8n^5-1}}$$


From the tests that I know of:


Divergence Test: The limit is ≠ to a constant, so inconclusive.


Geometric series: I don't think this could be written in that manner.


Comparison Test/Lim Comparison: Compare to $$\frac{n}{8n^{\frac{5}{3}}}$$


Integral Test: I can't think of a integration method that would work here.


Alternating Series/Root Test don't apply.


Ratio Test: The limit is 1 so inconclusive.


Perhaps I'm making a mistake throughout the methods I've tried, but I'm lost. Using these tests, is it possible to find whether or not it converges or diverges?



Answer



Use comparison test,$$\frac{n}{\sqrt[3]{8n^5-1}} \ge \frac{n}{\sqrt[3]{8n^5}} = \frac{1}{2n^{\frac53-1}}=\frac{1}{2n^{\frac23}}$$


Now, use $p$-series to make conclusion that it diverges.


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