Saturday, 27 May 2017

sequences and series - Can one show that sumNn=1frac1nlogNgammaleqslantfrac12N without using the Euler-Maclaurin formula?



I would like to prove that

Nn=11nlogNγ
without using the Euler-Maclaurin summation formula. The motivation for this is that I have come very close to doing so (see the answer provided below) but annoyingly have not actually proved the above.



Some may ask why I don't just use the formula. I'm writing a set of analytic number theory notes for my own use and it seems an unwieldy result to introduce and prove, given that the above inequality is all I need, and given that I have gotten so close without using Euler-Maclaurin!


Answer



Let
\gamma_n = \sum_{k=1}^n \frac{1}{k} - \log n.
Our goal is to show that

\gamma_n - \lim_{m \to \infty} \gamma_m \leq \frac{1}{2n}.
It is enough to show that, for $n\gamma_n - \gamma_m \leq \frac{1}{2n}.
This has the advantage of dealing solely with finite quantities.



Now,
\gamma_n - \gamma_m = \int_{n}^m \frac{dt}{t} - \sum_{k=n+1}^m \frac{1}{k} =\sum_{j=n}^{m-1} \int_{j}^{j+1} \left( \frac{1}{t} - \frac{1}{j+1} \right) \cdot dt .



At this point, if I were at a chalkboard rather than a keyboard, I would draw a picture. Draw the hyperbola y=1/x and mark off the interval between x=n and x=m. Divide this into m-n vertical bars of width 1. Each bar stretches up to touch the hyperbola at its right corner. There is a little wedge, bounded by x=j, y=1/(j+1) and y=1/x. We are adding up the area of each of these wedges.1




Because y=1/x is convex, the area of this wedge is less than that of the right triangle with vertices at (j,1/(j+1)), (j+1, 1/(j+1)) and (j,1/j). This triangle has base 1 and height 1/j - 1/(j+1), so its area is (1/2) (1/j - 1/(j+1)). So the quantity of interest is
\leq \sum_{j=n}^{m-1} \frac{1}{2} \left( \frac{1}{j} - \frac{1}{j+1} \right) = \frac{1}{2} \left( \frac{1}{n} - \frac{1}{m} \right) \leq \frac{1}{2n}.



Of course, this is just a standard proof of Euler-Maclaurin summation, but it is a lot more geometric and easy to follow in this special case.



1 By the way, since this area is positive, we also get the corollary that \gamma_n - \gamma_m > 0, so \gamma_n - \gamma >0, another useful bound.


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