Mertens function explained

In number theory, the Mertens function is defined for all positive integers n as

M(n) = \sum ^{n} _{k =1} \mu(k),

where

\mu(k) is the Möbius function. The function is named in honour of Franz Mertens. This definition can be extended to positive real numbers as follows:M(x) = M(\lfloor x \rfloor).

Less formally,

M(x) is the count of square-free integers up to x that have an even number of prime factors, minus the count of those that have an odd number.

The first 143 M(n) values are

M(n)	+0	+1	+2	+3	+4	+5	+6	+7	+8	+9	+10	+11
0+		1	0	-1	-1	-2	-1	-2	-2	-2	-1	-2
12+	-2	-3	-2	-1	-1	-2	-2	-3	-3	-2	-1	-2
24+	-2	-2	-1	-1	-1	-2	-3	-4	-4	-3	-2	-1
36+	-1	-2	-1	0	0	-1	-2	-3	-3	-3	-2	-3
48+	-3	-3	-3	-2	-2	-3	-3	-2	-2	-1	0	-1
60+	-1	-2	-1	-1	-1	0	-1	-2	-2	-1	-2	-3
72+	-3	-4	-3	-3	-3	-2	-3	-4	-4	-4	-3	-4
84+	-4	-3	-2	-1	-1	-2	-2	-1	-1	0	1	2
96+	2	1	1	1	1	0	-1	-2	-2	-3	-2	-3
108+	-3	-4	-5	-4	-4	-5	-6	-5	-5	-5	-4	-3
120+	-3	-3	-2	-1	-1	-1	-1	-2	-2	-1	-2	-3
132+	-3	-2	-1	-1	-1	-2	-3	-4	-4	-3	-2	-1

The Mertens function slowly grows in positive and negative directions both on average and in peak value, oscillating in an apparently chaotic manner passing through zero when n has the values

2, 39, 40, 58, 65, 93, 101, 145, 149, 150, 159, 160, 163, 164, 166, 214, 231, 232, 235, 236, 238, 254, 329, 331, 332, 333, 353, 355, 356, 358, 362, 363, 364, 366, 393, 401, 403, 404, 405, 407, 408, 413, 414, 419, 420, 422, 423, 424, 425, 427, 428, ... .

Because the Möbius function only takes the values -1, 0, and +1, the Mertens function moves slowly, and there is no x such that |M(x)| > x. H. Davenport^[1] demonstrated that, for any fixed h,

\sum ^{x} _{n =1} \mu(n)\exp(i 2 \pi n\theta) = O\left( (x)/(log ^{hx}) \right)

uniformly in

\theta . This implies, for \theta =0 thatM(x) = O\left( (x)/(log ^{hx}) \right) .

The Mertens conjecture went further, stating that there would be no x where the absolute value of the Mertens function exceeds the square root of x. The Mertens conjecture was proven false in 1985 by Andrew Odlyzko and Herman te Riele. However, the Riemann hypothesis is equivalent to a weaker conjecture on the growth of M(x), namely M(x) = O(x^{1/2 + ε}). Since high values for M(x) grow at least as fast as

\sqrt{x} , this puts a rather tight bound on its rate of growth. Here, O refers to big O notation.

The true rate of growth of M(x) is not known. An unpublished conjecture of Steve Gonek states that

0 < \limsup _{x} (|M(x)|)/(\sqrt{x) ( log log log x) ^{5/4} } < \infty.

Probabilistic evidence towards this conjecture is given by Nathan Ng.^[2] In particular, Ng gives a conditional proof that the function

e ^{-y/2} M(e ^{y)} has a limiting distribution \nu on R . That is, for all bounded Lipschitz continuous functions f on the reals we have that\lim _{Y\toinfty} (1)/(Y) \int ^{Y} _{0} f ( e ^{-y/2} M(e ^{y))} dy = \int ^{inf ty} _{-inf ty} f(x) d\nu(x),

if one assumes various conjectures about the Riemann zeta function.

Representations

As an integral

Using the Euler product, one finds that

(1)/(\zeta(s)) = \prod _{p} ( 1 - p ^{-s} ) = \sum ^{inf ty} _{n =1} (\mu(n))/(n ^{s}) ,

where

\zeta(s) is the Riemann zeta function, and the product is taken over primes. Then, using this Dirichlet series with Perron's formula, one obtains(1)/(2 \pi i) \int ^{c + i inf ty} _{c - i inf ty} (x ^{s})/(s \zeta(s)) ds = M(x),

where c > 1.

Conversely, one has the Mellin transform

(1)/(\zeta(s)) = s \int ^{inf ty} _{1} (M(x))/(x ^{s+1}) dx,

which holds for

\operatorname{Re}(s) > 1 .

A curious relation given by Mertens himself involving the second Chebyshev function is

\psi(x) = M\left( (x)/(2) \right) log 2 + M \left( (x)/(3) \right) log 3 + M \left( (x)/(4) \right ) log 4 + … .

Assuming that the Riemann zeta function has no multiple non-trivial zeros, one has the "exact formula" by the residue theorem:

M(x) = \sum _{\rho} (x ^{\rho})/(\rho \zeta'(\rho)) - 2 + \sum ^{inf ty} _{n =1} (( -1 ) ^{n-1} ( 2 \pi) ^{2n})/(( 2 n)! n \zeta( 2 n +1 ) x ^{2n}) .

Weyl conjectured that the Mertens function satisfied the approximate functional-differential equation

(y(x))/(2) - \sum ^{N} _{r =1} (B _{2r})/(( 2 r)!) D ^{2 r -1} _{t} y \left( (x)/(t +1) \right) + x \int ^{x} _{0} (y(u))/(u ^{2}) du = x ^{-1} H( log x),

where H(x) is the Heaviside step function, B are Bernoulli numbers, and all derivatives with respect to t are evaluated at t = 0.

There is also a trace formula involving a sum over the Möbius function and zeros of the Riemann zeta function in the form

\sum ^{inf ty} _{n =1} (\mu(n))/(\sqrt{n) } g(\log n) = \sum_\gamma \frac + 2 \sum_^\infty \frac \int_^\infty g(x) e^ \, dx,

where the first sum on the right-hand side is taken over the non-trivial zeros of the Riemann zeta function, and (g, h) are related by the Fourier transform, such that

2 \pi g(x) = \int ^{inf ty} _{-inf ty} h(u) e ^{iux} du.

As a sum over Farey sequences

Another formula for the Mertens function is

M(n) = -1 + \sum _{a\inl{F} _{n}} e ^{2\pi} ,

where

l{F} _{n} is the Farey sequence of order n.

This formula is used in the proof of the Franel–Landau theorem.^[3]

As a determinant

M(n) is the determinant of the n × n Redheffer matrix, a (0, 1) matrix in whicha_ij is 1 if either j is 1 or i divides j.

As a sum of the number of points under n-dimensional hyperboloids

M(x) = 1 - \sum _{2} 1 + \underset{ab \leq x}{\sum _{a} \sum _{b} } 1 - \underset 1 + \underset 1 - \cdots

This formulation expanding the Mertens function suggests asymptotic bounds obtained by considering the Piltz divisor problem, which generalizes the Dirichlet divisor problem of computing asymptotic estimates for the summatory function of the divisor function.

Other properties

From ^[4] we have

\sum ^{n} _{d =1} M(\lfloor n/d \rfloor) = 1 .

Furthermore, from ^[5]

\sum ^{n} _{d =1} M(\lfloor n/d \rfloor)d = \Phi(n) , where \Phi(n) is the totient summatory function.

Calculation

Neither of the methods mentioned previously leads to practical algorithms to calculate the Mertens function.Using sieve methods similar to those used in prime counting, the Mertens function has been computed for all integers up to an increasing range of x.^{[6] [7]}

Person	Year	Limit
Mertens	1897	10^4
von Sterneck	1897	1.5
von Sterneck	1901	5
von Sterneck	1912	5
Neubauer	1963	10^8
Cohen and Dress	1979	7.8
Dress	1993	10^12
Lioen and van de Lune	1994	10^13
Kotnik and van de Lune	2003	10^14
Hurst	2016	10^16

The Mertens function for all integer values up to x may be computed in time. A combinatorial algorithm has been developed incrementally starting in 1870 by Ernst Meissel,^[8] Lehmer,^[9] Lagarias-Miller-Odlyzko,^[10] and Deléglise-Rivat^[11] that computes isolated values of M(x) in time; a further improvement by Harald Helfgott and Lola Thompson in 2021 improves this to,^[12] and an algorithm by Lagarias and Odlyzko based on integrals of the Riemann zeta function achieves a running time of .^[13]

See for values of M(x) at powers of 10.

Known upper bounds

Ng notes that the Riemann hypothesis (RH) is equivalent to

M(x) = O\left(\sqrt{x} \exp\left( (C ⋅ log x)/(loglog x) \right)\right),

for some positive constant

C> 0 . Other upper bounds have been obtained by Maier, Montgomery, and Soundarajan assuming the RH including\begin{align} |M(x)| & \ll \sqrt{x} \exp\left(C _{2} ⋅ ( log x) ^{(39)/(61)} \right) \ |M(x)| & \ll \sqrt{x} \exp\left(\sqrt{ log x} ( loglog x) ^{14} \right). \end{align}

Known explicit upper bounds without assuming the RH are given by:^[14]

\begin{align} |M(x)| & < (12590292 ⋅ x)/(log ^{236/75} (x)) , for x>\exp( 12282 . 3 ) \ |M(x)| & < (0 . 6437752 ⋅ x)/(log x) , for x> 1 . \end{align}

It is possible to simplify the above expression into a less restrictive but illustrative form as:

\begin{align} M(x) = O \left( (x)/(log ^{\pi} (x)) \right) . \end{align}

See also

• Perron's formula
• Liouville's function

References

• Book: Edwards , Harold . Harold Edwards (mathematician) . Riemann's Zeta Function . Dover . Mineola, New York . 1974 . 0-486-41740-9.
• Mertens . F. . 1897 . "Über eine zahlentheoretische Funktion", Akademie Wissenschaftlicher Wien Mathematik-Naturlich . Kleine Sitzungsber, IIA . 106 . 761–830 .
• Odlyzko . A. M. . A. M. Odlyzko . Herman te Riele . te Riele . Herman . 1985 . Disproof of the Mertens Conjecture . Journal für die reine und angewandte Mathematik . 357 . 138–160 .
  • A002321. Mertens's function.
• Deléglise, M. and Rivat, J. "Computing the Summation of the Möbius Function." Experiment. Math. 5, 291-295, 1996. Computing the summation of the Möbius function
• Hurst . Greg . 2016 . Computations of the Mertens Function and Improved Bounds on the Mertens Conjecture . 1610.08551 . math.NT.
• Nathan Ng, "The distribution of the summatory function of the Möbius function", Proc. London Math. Soc. (3) 89 (2004) 361-389. http://www.cs.uleth.ca/~nathanng/RESEARCH/mobius2b.pdf

Notes and References

1. Davenport . H.. On Some Infinite Series Involving Arithmetical Functions (Ii) . The Quarterly Journal of Mathematics . Original Series . 8 . 1 . 313–320 . November 1937. 10.1093/qmath/os-8.1.313 .
2. The distribution of the summatory function of the Mobius function. Nathan Ng. October 25, 2018. math/0310381 .
3. Edwards, Ch. 12.2.
4. Lehman . R.S. . On Liouville's Function . Math. Comput. . 14 . 311–320 . 1960.
5. Kanemitsu . S. . Yoshimoto. M. . Farey series and the Riemann hypothesis . Acta Arithmetica . 1996. 75 . 4 . 351–374 . 10.4064/aa-75-4-351-374 . free .
6. Web site: Kotnik . Tadej . van de Lune . Jan . Further systematic computations on the summatory function of the Möbius function . Modelling, Analysis and Simulation . MAS-R0313. November 2003.
7. Hurst . Greg . 2016 . Computations of the Mertens Function and Improved Bounds on the Mertens Conjecture . 1610.08551 . math.NT.
8. Meissel . Ernst . 1870 . Ueber die Bestimmung der Primzahlenmenge innerhalb gegebener Grenzen . Mathematische Annalen . de . 2 . 4 . 636–642 . 10.1007/BF01444045 . 119828499 . 0025-5831.
9. Lehmer . Derrick Henry . April 1, 1958 . ON THE EXACT NUMBER OF PRIMES LESS THAN A GIVEN LIMIT . Illinois J. Math. . 3 . 3 . 381–388 . February 1, 2017 .
10. Lagarias . Jeffrey . Miller . Victor . Odlyzko . Andrew . April 11, 1985 . Computing \pi(x) : The Meissel–Lehmer method . Mathematics of Computation . 44 . 170 . 537–560 . 10.1090/S0025-5718-1985-0777285-5 . September 13, 2016 . free .
11. Rivat . Joöl . Deléglise . Marc . 1996 . Computing the summation of the Möbius function . Experimental Mathematics . en . 5 . 4 . 291–295 . 10.1080/10586458.1996.10504594 . 574146 . 1944-950X.
12. Helfgott . Harald . Thompson . Lola . Summing \mu(n) : a faster elementary algorithm . Research in Number Theory . 2023 . 9 . 1 . 6 . 2363-9555 . 10.1007/s40993-022-00408-8. 36511765 . 9731940 .
13. Lagarias . Jeffrey . Odlyzko . Andrew . Computing \pi(x) : An analytic method . Journal of Algorithms . June 1987 . 8 . 2 . 173–191 . 10.1016/0196-6774(87)90037-X.
14. El Marraki . M. . Fonction sommatoire de la fonction de Möbius, 3. Majorations asymptotiques effectives fortes . Journal de théorie des nombres de Bordeaux. 1995 . 7 . 2 . 16 April 2023 --> .