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Less formally, M(n) is the count of square-free integers up to n that have an even number of prime factors, minus the count of those that have an odd number.
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, ... (sequence A028442 in OEIS).
Because the Möbius function only takes the values −1, 0, and +1, the Mertens function moves slowly and there is no n such that |M(n)| > n. The Mertens conjecture went further, stating that there would be no n where the absolute value of the Mertens function exceeds the square root of n. 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(n), namely M(n) = O(n1/2 + ε). Since high values for M(n) grow at least as fast as the square root of n, this puts a rather tight bound on its rate of growth. Here, O refers to Big O notation.
The above definition can be extended to real numbers as follows:
As an integral
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Using the Euler product one finds that
where c > 1.
Conversely, one has the Mellin transform
which holds for .
A curious relation given by Mertens himself involving the second Chebyshev function is
A good evaluation, at least asymptotically, would be to obtain, by the method of steepest descent, the inequality
Assuming that there are not multiple non-trivial roots of we have the "exact formula" by the residue theorem:
Weyl conjectured that the Mertens function satisfied the approximate functional-differential equation
Titchmarsh(1960) provided a Trace formula involving a sum over the Möbius function and zeros of Riemann Zeta in the form
where 't' sums over the imaginary parts of nontrivial zeros, and (g, h) are related by a Fourier transform, such that
As a sum over Farey sequences
Another formula for the Mertens function is
- where is the Farey sequence of order n.
As a determinant
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 an increasing range of n.
|Cohen and Dress||1979||7.8×109|
|Lioen and van de Lune||1994||1013|
|Kotnik and van de Lune||2003||1014|
The Mertens function for all integer values up to N may be computed in O(N2/3+ε) time, while better methods are known. Elementary algorithms exist to compute isolated values of M(N) in O(N2/3*(ln ln(N))1/3) time.
See A084237 for values of M(N) at powers of 10.
- Edwards, Ch. 12.2
- Edwards, Harold (1974). Riemann's Zeta Function. Mineola, New York: Dover. ISBN 0-486-41740-9.
- F. Mertens, "Über eine zahlentheoretische Funktion", Akademie Wissenschaftlicher Wien Mathematik-Naturlich Kleine Sitzungsber, IIa 106, (1897) 761–830.
- A. M. Odlyzko and Herman te Riele, "Disproof of the Mertens Conjecture", Journal für die reine und angewandte Mathematik 357, (1985) pp. 138–160.
- Weisstein, Eric W., "Mertens function", MathWorld.
- "Sloane's A002321 : Mertens's function", The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
- Deléglise, M. and Rivat, J. "Computing the Summation of the Möbius Function." Experiment. Math. 5, 291-295, 1996. http://projecteuclid.org/DPubS/Repository/1.0/Disseminate?view=body&id=pdf_1&handle=euclid.em/1047565447