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What is $\sum_{i=1}^n |\{k \in \mathbb{N} \mid k | i\}|$ asymptotically (as a function of $n$)?

(I'm summing, for each of $1,\dotsc,n$, its number of divisors)

Or at least, what's the best upper bound you can find?

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The number of divisors of $n$ is usually denoted $d(n)$ or $\sigma_0(n)$. –  Arturo Magidin Aug 16 '11 at 17:55
    
Does it behave like harmonic sum? –  Goog Aug 16 '11 at 17:56
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2 Answers 2

up vote 12 down vote accepted

Let $d(n)$ denote the number of divisors of $n$.

Dirichlet's Asymptotic Formula. (Theorem 3.3 in Tom Apostol's Introduction to Analytic Number Theory) For all $x\geq 1$, we have $$\sum_{n\leq x} d(n) = x \log x + (2C-1)x + O(\sqrt{x}),$$ where $C$ is Euler's constant, $$C = \lim_{n\to\infty}\left(1 + \frac{1}{2} + \frac{1}{3}+\cdots+\frac{1}{n}-\log n\right).$$

The proof involves writing the sum as $$\sum_{n\leq x}d(n) = \sum_{d\leq x} \sum_{q\leq x/d} 1,$$ and interpreting it as counting lattice points in the $qd$ plane; the lattice points with $qd=n$ lie on a hyperbola; then one can get that $$\sum_{q\leq x/d} 1 = \frac{x}{d} + O(1),$$ and from this and the fact that $$\sum_{n\leq x}n^{\alpha} = \frac{x^{\alpha+1}}{\alpha+1} + O(x^{\alpha}),\qquad \text{if }\alpha\geq 0,$$ one derives that $$\sum_{n\leq x}d(n) = x\log x + O(x).$$ The more precise formula uses the symmetry of the corresponding hyperbola along $q=d$. Apostol's book has all the details.

The error term $O(\sqrt{x})$ can be improved: you can see a summary in Wikipedia; best estimate so far is due to Huxley, who proved the error term is $O(x^{(131/416)+\epsilon})$ for all $\epsilon\gt 0$. Hardy and Landau proved that the smallest exponent possible is at least $\frac{1}{4}$.

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In terms of the lower bound and $\Omega$ results, the best known is due to Soundararajan and states that $$\Delta(x)=\Omega\left(\frac{x^\frac{1}{4}\left(\log x\right)^\frac{1}{4} (\log \log x)^\beta}{\left(\log \log \log x\right)^\frac{5}{8}}\right)$$ where $\alpha=\frac{3}{4}\left(2^\frac{4}{3}-1\right)$. Now, the very interesting thing is that "it is plausible the first three exponents above are optimal." (quote from Montgomery and Vaughns book, Chapter 2 Notes) I remember reading this quite a while ago and finding it very strange. See: imrn.oxfordjournals.org/content/2003/36/1987 –  Eric Naslund Aug 18 '11 at 0:29
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You can find a good summary in the following Wikipedia article.

The main term in the estimate for $\sum_{i=1}^n d(i)$ is $$n\log n +(2\gamma -1)n,$$ where $\gamma$ is Euler's $\gamma$.

There is a large literature on the error term. Dirichlet got $O(n^{1/2})$, and it is known that one cannot do better than $O(n^{1/4})$.

Note: For a number-theoretic function, this is a fantastically precise estimate. We have a dominant term, $n\log n$, that gives us the asymptotic behaviour. We have also an explicitly known correction term $(2\gamma-1)n$. And, by number-theoretic estimate standards, the error term is quite small compared to $(2\gamma-1)n$.

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