The classical Möbius function μ(n) is an important multiplicative function in number theory and combinatorics. The German mathematician August Ferdinand Möbius introduced it in 1832. This classical Möbius function is a special case of a more general object in combinatorics (see below).
- μ(n) = 1 if n is a square-free positive integer with an even number of prime factors.
- μ(n) = −1 if n is a square-free positive integer with an odd number of prime factors.
- μ(n) = 0 if n has a squared prime factor.
- 1, −1, −1, 0, −1, 1, −1, 0, 0, 1, −1, 0, −1, 1, 1, 0, −1, 0, −1, 0, 1, 1, −1, 0, 0, ...
The first 50 values of the function are plotted below:
Properties and applications
(A consequence of the fact that every non-empty finite set has just as many subsets with odd numbers of elements as subsets with even numbers of elements – in the same way as binomial coefficients exhibit alternating entries of odd and even power which sum symmetrically.) This leads to the important Möbius inversion formula and is the main reason why μ is of relevance in the theory of multiplicative and arithmetic functions.
Other applications of μ(n) in combinatorics are connected with the use of the Pólya enumeration theorem in combinatorial groups and combinatorial enumerations.
for every natural number n. This function is closely linked with the positions of zeroes of the Riemann zeta function. See the article on the Mertens conjecture for more information about the connection between M(n) and the Riemann hypothesis.
The ordinary generating function for the Möbius function follows from the binomial series
applied to triangular matrices:
The Lambert series for the Möbius function is:
This is easy to see from its Euler product
There is a formula for calculating the Möbius function without directly knowing the factorization of its argument:
i.e. μ(n) is the sum of the primitive nthroots of unity. (However, the computational complexity of this definition is at least the same as of the Euler Product definition.)
From this it follows that the Mertens function is given by:
- where is the Farey sequence of order n.
The infinite symmetric matrix starting:
defined by the recurrence:
where "a" is the Dirichlet inverse of the Euler totient function,
can be used to calculate the Möbius function:
The Dirichlet series for the Mobius function has the equivalence:
- 4, 8, 9, 12, 16, 18, 20, 24, 25, 27, 28, 32, 36, 40, 44, 45, 48, 49, 50, 52, 54, 56, 60, 63,....
If n is prime, then μ(n) = −1, but the converse is not true. The first non prime n for which μ(n) = −1 is 30 = 2·3·5. The first such numbers with three distinct prime factors (sphenic numbers) are:
- 30, 42, 66, 70, 78, 102, 105, 110, 114, 130, 138, 154, 165, 170, 174, 182, 186, 190, 195, 222, … (sequence A007304 in OEIS).
and the first such numbers with 5 distinct prime factors are:
In combinatorics, every locally finite partially ordered set (poset) is assigned an incidence algebra. One distinguished member of this algebra is that poset's "Möbius function". The classical Möbius function treated in this article is essentially equal to the Möbius function of the set of all positive integers partially ordered by divisibility. See the article on incidence algebras for the precise definition and several examples of these general Möbius functions.
The Möbius function also arises in the primon gas or free Riemann gas model of supersymmetry. In this theory, the fundamental particles or "primons" have energies log p. Under second quantization, multiparticle excitations are considered; these are given by log n for any natural number n. This follows from the fact that the factorization of the natural numbers into primes is unique.
In the free Riemann gas, any natural number can occur, if the primons are taken as bosons. If they are taken as fermions, then the Pauli exclusion principle excludes squares. The operator (−1)F that distinguishes fermions and bosons is then none other than the Möbius function μ(n).
The free Riemann gas has a number of other interesting connections to number theory, including the fact that the partition function is the Riemann zeta function. This idea underlies Alain Connes' attempted proof of the Riemann hypothesis.
- Hardy & Wright, Notes on ch. XVI: "... μ(n) occurs implicitly in the works of Euler as early as 1748, but Möbius, in 1832, was the first to investigate its properties systematically."
- In the Disquisitiones Arithmeticae (1801) Karl Friedrich Gauss showed that the sum of the primitive roots (mod p) is μ(p − 1), (see #Properties and applications) but he didn't make further use of the function. In particular, he didn't use Möbius inversion in the Disquisitiones.
- Hardy & Wright 1980, (16.6.4), p. 239
- Edwards, Ch. 12.2
- Gauss, Disquisitiones, Art. 81
- Jacobson 2009, §4.13
- Mats Granvik, Is this sum equal to the Möbius function? (2011)
- Apostol 1976, §3.9
- J.-B. Bost and Alain Connes (1995), "Hecke Algebras, Type III factors and phase transitions with spontaneous symmetry breaking in number theory", Selecta Math. (New Series), 1 411-457.
The Disquisitiones Arithmeticae has been translated from Latin into English and German. The German edition includes all of his papers on number theory: all the proofs of quadratic reciprocity, the determination of the sign of the Gauss sum, the investigations into biquadratic reciprocity, and unpublished notes.
- Gauss, Carl Friedrich (1986), Disquisitiones Arithemeticae, Arthur A. Clarke (English translator) (corrected 2nd ed.), New York: Springer, ISBN 0-387-96254-9
- Gauss, Carl Friedrich (1965), Untersuchungen uber hohere Arithmetik (Disquisitiones Arithemeticae & other papers on number theory), H. Maser (German translator) (2nd ed.), New York: Chelsea, ISBN 0-8284-0191-8
- Apostol, Tom M. (1976), Introduction to analytic number theory, Undergraduate Texts in Mathematics, New York-Heidelberg: Springer-Verlag, ISBN 978-0-387-90163-3, MR 0434929, Zbl 0335.10001
- Computing the summation of the Möbius function by Marc Deléglise and Joël Rivat Experimental Mathematics Volume 5, Issue 4291-295
- Edwards, Harold (1974), Riemann's Zeta Function, Mineola, New York: Dover, ISBN 0-486-41740-9
- Hardy, G. H.; Wright, E. M. (1980), An Introduction to the Theory of Numbers (5th ed.), Oxford: Oxford University Press, ISBN 978-0-19-853171-5
- Jacobson, Nathan (2009) , Basic algebra I (2nd ed.), Dover Publications, ISBN 978-0-486-47189-1
- Sándor, József; Mitrinović, Dragoslav S.; Crstici, Borislav, eds. (2006), Handbook of number theory I, Dordrecht: Springer-Verlag, pp. 187–226, ISBN 1-4020-4215-9, Zbl 1151.11300
- N.I. Klimov (2001), "Möbius function", in Hazewinkel, Michiel, Encyclopedia of Mathematics, Springer, ISBN 978-1-55608-010-4
- Ed Pegg, Jr., "The Möbius function (and squarefree numbers)", MAA Online Math Games (2003)
- Weisstein, Eric W., "Möbius function", MathWorld.