Tuesday, 12 June 2018

real analysis - divergence of $sum_{n=3}^infty frac{sqrt{n}+2}{n-2}$ verification/ alternative method



I wish to prove divergence of
$$\sum_{n=3}^\infty \frac{\sqrt{n}+2}{n-2}$$




I wish to do so by comparison, since $n\geq 3$:
$$\sum_{n=3}^\infty \frac{\sqrt{n}+2}{n-2} > \sum_{n=3}^\infty \frac{1+2}{n-2}>\sum_{n=3}^\infty \frac{3}{n}>\sum_{n=3}^\infty \frac{1}{n} \rightarrow \infty$$
And the harmonic series is divergent, so if we just remove finitely many terms, we still have that it is divergent, because divergence is determined "in the tail". We have a divergent minorant series and hence the original series diverges to $\infty$.



Is this approach fine, or is there some more elegant method, this was about the simplest thing I could think of.






Alternatively we have:
$$\sum_{n=3}^\infty \frac{\sqrt{n}+2}{n-2} > \sum_{n=3}^\infty \frac{\sqrt{n}+2}{n}=\sum_{n=3}^\infty \frac{1}{\sqrt{n}}+ \frac{2}{n}\rightarrow \infty$$



Answer



Not clear what your question is, but your answer is correct.



Your approach is fine. Comparison test would be the proper test to use.



One can also show that $$\sum _{n=3}^{\infty \:}\frac{\sqrt{n}+2}{n-2}\ge \sum _{n=3}^{\infty \:}\frac{\sqrt{n}+2}{n}$$



and show that the rightmost sum is diverging via the integral test.


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