Monday, 15 August 2016

sequences and series - Does sumin=1nftyfrac(1)nn1+frac1n converge?



I want to use the alternating series test here, but I've just been told that it won't work because it's not monotonically decreasing.



However, if the alternating harmonic series converges then don't we have for n=1(1)nn1+1n that
lim
since
\lim_{n \to \infty} \frac{1}{n^{1+\frac{1}{n}}} < \lim_{n \to \infty} \frac{1}{n} = 0.




Can someone point out where the mistake here is?


Answer



To show that it is monotonically decreasing one should show that:
\frac{1}{n^{1+\frac{1}{n}}} > \frac{1}{(n+1)^{1+\frac{1}{n+1}}}.
This is equivalent to showing that:
\frac{n+1}{n} > \frac{n^\frac{1}{n}}{(n+1)^\frac{1}{n+1}},
which is the same as
(1+\frac{1}{n})^n > \frac{n}{n+1}\cdot (n+1)^\frac{1}{n+1}.




For sufficiently large values of n, this must be the case, as the limit of the LHS is just e and the one of the RHS is 1.


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