# Mertens conjecture

In mathematics, the Mertens conjecture is the disproven statement that the Mertens function $M(n)$ is bounded by ${\sqrt {n}}$ , which implies the Riemann hypothesis. It was conjectured by Thomas Joannes Stieltjes in an 1885 letter to Charles Hermite (reprinted in Stieltjes (1905)) and Franz Mertens (1897), and disproved by Andrew Odlyzko and Herman te Riele (1985). It is a striking example of a mathematical proof contradicting a large amount of computational evidence in favor of a conjecture.

## Definition

In number theory, we define the Mertens function as

$M(n)=\sum _{1\leq k\leq n}\mu (k)$ where μ(k) is the Möbius function; the Mertens conjecture is that for all n > 1,

$\left|M(n)\right|<{\sqrt {n}}.\,$ ## Disproof of the conjecture

Stieltjes claimed in 1885 to have proven a weaker result, namely that $m(n)=M(n)/{\sqrt {n}}$ was bounded, but did not publish a proof. (In terms of $m(n)$ , the Mertens conjecture is that $-1 .)

In 1985, Andrew Odlyzko and Herman te Riele proved the Mertens conjecture false: indeed, $\liminf m(n)<-1.009$ and $\limsup m(n)>1.06$ . It was later shown that the first counterexample appears below $e^{3.21\times 10^{64}}\approx 10^{1.39\times 10^{64}}$ (Pintz 1987) but above 1014. The upper bound has since been lowered to $e^{1.59\times 10^{40}}$ (Kotnik and Te Riele 2006) or approximately $10^{6.91\times 10^{39}}$ , but no counterexample is explicitly known. The law of the iterated logarithm states that if μ is replaced by a random sequence of +1s and 1s then the order of growth of the partial sum of the first n terms is (with probability 1) about (n log log n)1/2, which suggests that the order of growth of m(n) might be somewhere around (log log n)1/2. The actual order of growth may be somewhat smaller; it was conjectured by Steve Gonek in the early 1990s that the order of growth of m(n) was $(\log {\log {\log {n}}})^{5/4}$ , which was also conjectured by Ng (2004), based on a heuristic argument assuming the Riemann hypothesis and certain conjectures about the averaged behavior of zeros of the Riemann zeta function.

In 1979, Cohen and Dress found the largest known value of $m(n)\approx 0.570591$ for M(7766842813) = 50286, and, in 2011, Kuznetsov found the largest known negative value $m(n)\approx -0.585768$ for M(11609864264058592345) = -1995900927. In 2016, Hurst computed M(n) for every n ≤ 1016 but did not find larger values of m(n).

In 2006, Kotnik and te Riele improved the upper bound and showed that there are infinitely many values of n for which m(n)>1.2184, but without giving any specific value for such an n. In 2016, Hurst made further improvements by showing $\liminf m(n)<-1.837625$ and $\limsup m(n)>1.826054$ .

## Connection to the Riemann hypothesis

The connection to the Riemann hypothesis is based on the Dirichlet series for the reciprocal of the Riemann zeta function,

${\frac {1}{\zeta (s)}}=\sum _{n=1}^{\infty }{\frac {\mu (n)}{n^{s}}},$ valid in the region $\Re (s)>1$ . We can rewrite this as a Stieltjes integral

${\frac {1}{\zeta (s)}}=\int _{0}^{\infty }x^{-s}\,dM(x)$ and after integrating by parts, obtain the reciprocal of the zeta function as a Mellin transform

${\frac {1}{s\zeta (s)}}=\left\{{\mathcal {M}}M\right\}(-s)=\int _{0}^{\infty }x^{-s}M(x)\,{\frac {dx}{x}}.$ Using the Mellin inversion theorem we now can express M in terms of 1/ζ as

$M(x)={\frac {1}{2\pi i}}\int _{\sigma -i\infty }^{\sigma +i\infty }{\frac {x^{s}}{s\zeta (s)}}\,ds$ which is valid for 1 < σ < 2, and valid for 1/2 < σ < 2 on the Riemann hypothesis. From this, the Mellin transform integral must be convergent, and hence M(x) must be O(xe) for every exponent e greater than 1/2. From this it follows that

$M(x)=O(x^{{\frac {1}{2}}+\epsilon })$ for all positive ε is equivalent to the Riemann hypothesis, which therefore would have followed from the stronger Mertens hypothesis, and follows from the hypothesis of Stieltjes that

$M(x)=O(x^{\frac {1}{2}})$ .