# Growth rate of a divisor function

https://en.wikipedia.org/wiki/Divisor_function#CITEREFHardyWright2008

I was wondering about a formula which appear under the Growth rate section :

for all $$\varepsilon>0$$, $$d(n)=\mathcal{O}(n^\epsilon)$$ where $$d(n)$$ stands for number divisors $$n$$ has.

I would like to know why this is true?

• The wikipedia article uses o, little o, not O, big O. Both are true. – Ross Millikan Dec 16 '18 at 21:41
• @WesleyStrik sorry I eddited it..I would like to know why this is true? – נירייב שמואל Dec 16 '18 at 21:42
• @RossMillikan I would like to know why this is true? I edited the page sorry. – נירייב שמואל Dec 16 '18 at 21:43
• Basically to get lots of divisors you either need lots of prime factors and the primes get bigger or you need lots of factors of a small prime, but the number of divisors grows very slowly (logarithmically) with the number of factors. Either way you need an enormous number to have lots of factors. – Ross Millikan Dec 16 '18 at 21:44
• The formulas quoted in Will Jagy's answer show that $d(n)\lt n^\epsilon$ for $n$ large enough and any given $\epsilon \gt 0$ – Ross Millikan Dec 16 '18 at 21:52

It's true. There are many interesting inequalities involving $$d(n)$$
I will start off with the simplest type, $$d(n) \leq \sqrt{3 n}$$ and $$d(n) \leq 48 \left(\frac{n}{2520}\right)^{1/3}$$ and $$d(n) \leq 576 \left(\frac{n}{21621600}\right)^{1/4}.$$ The first one has equality only at $$n = 12,$$ second only at $$n =2520,$$ third only at $$n= 21621600.$$ Instead of continuing with fractional powers $$1/k$$ the better results switch to logarithms. Reference is a 1988 paper by J. L. Nicolas in a book called Ramanujan Revisited.
With equality at $$n = 6983776800 = 2^5 \cdot 3^3 \cdot 5^2 \cdot 7 \cdot 11 \cdot 13 \cdot 17 \cdot 19$$ and $$d(n) = 2304,$$ $$d(n) \leq n^{ \left( \frac{\log 2}{\log \log n} \right) \left( 1.5379398606751... \right)} = n^{ \left( \frac{1.0660186782977...}{\log \log n} \right) }.$$ Full details of the proof appear in J.-L. Nicolas et G. Robin. Majorations explicites pour le nombre de diviseurs de n, Canad. Math. Bull., 26, 1983, 485--492. The next two appear in the dissertation of Robin, are repeated in the 1988 Nicolas survey article indicated.
With equality at a number $$n$$ near $$6.929 \cdot 10^{40},$$ $$d(n) \leq n^{ \left( \frac{\log 2}{\log \log n} \right) \left( 1 + \frac{1.934850967971...}{\log \log n} \right)}.$$ Compare this one with Theorem 317 in Hardy and Wright, attributed to Wigert (1907), $$\limsup \frac{\log d(n) \log \log n}{\log n} = \log 2.$$
With equality at a number $$n$$ near $$3.309 \cdot 10^{135},$$ $$d(n) \leq n^{ \left( \frac{\log 2}{\log \log n} \right) \left( 1 + \frac{1}{\log \log n} + \frac{4.762350121177...}{\left(\log \log n \right)^2} \right)}$$
• no, the more specific $$d(n) \leq n^{ \left( \frac{1.0660186782977...}{\log \log n} \right) }.$$ – Will Jagy Dec 16 '18 at 21:46