# Möbius inversion

A method for inverting sums over partially ordered sets (or posets; cf. also Partially ordered set). The theory of Möbius inversion matured in the classic paper [a4] of G.-C. Rota and is a cornerstone of algebraic combinatorics (cf. also Combinatorics).

Let be a locally finite partially ordered set, that is, a poset in which every interval is finite. The Möbius function of is the function on pairs of elements in defined by the following conditions: if , for all , and the recursive relation

When is finite, the function value is the -entry in the inverse of the incidence matrix of the partial order relation on .

Let , be functions defined from to a commutative ring with unity. The Möbius inversion formula states:

or, dually,

For the Boolean algebra of all subsets of a set , when . When is thought of as a set of "properties" , the Möbius inversion formula yields the principle of inclusion-exclusion (cf. Inclusion-and-exclusion principle).

When is the distributive lattice of positive integers ordered by divisibility, the Möbius function equals unless divides and is square-free, in which case it equals , where is the number of primes in the prime factorization of . The classical or number-theoretic Möbius inversion formula states:

For example, this yields the following formula for the Euler totient function , the number of positive integers not exceeding that are relatively prime to (cf. also Euler function):

The Möbius function satisfies many identities. Four often-used identities are:

a) The P. Hall identity:

where is the number of (strict) chains with links starting at and ending at (cf. also Chain).

b) The Weisner theorem: Let be a finite lattice with minimum . If and are elements of such that , then

the sum ranging over all elements such that and .

c) The Crapo complementation theorem: An element is a complement of an element in a lattice with minimum and maximum if and . For any element in a finite lattice ,

the sum ranging over all pairs and such that and both and are complements of .

d) The Boolean expansion lemma: If is a closure operator on a set (cf. also Closure space), then for a closed set in the lattice of closed sets,

The Boolean expansion lemma is a special case of the Galois connection theorem. It is also a special case of the cross-cut theorem. A cross-cut in a finite lattice is a set of elements of satisfying:

1) does not contain the minimum or the maximum

2) no pair of elements of is comparable;

3) any maximal chain from to has non-empty intersection with . A subset of elements of is spanning if both the join of all the elements in is and the meet of all the elements in is . If is a finite lattice with more than two elements and is a cross-cut in , then

where is the number of spanning subsets of with elements.

Besides order-theoretic, homological and counting proofs of these results, there are also proofs using the Möbius algebra, a generalization of the Burnside algebra of a group.

Much work has been done on calculating Möbius functions of specific partially ordered sets. For example, if and are subspaces in the lattice of subspaces of a finite-dimensional vector space over a finite field of order and , then

where is the difference .

There are many results relating structural properties and properties of the Möbius function. Two examples follow. For an element in a lattice, only if the element is a join of atoms. (Atoms are elements covering the minimum ; cf. also Atom.) If is a finite lattice in which is non-zero for all elements , then there exists a bijection from to itself such that for every , is the maximum .

Möbius functions occur in many proofs. For example, they are heavily used in the original proof of Dilworth's theorem that in a finite modular lattice, the number of elements covering or fewer elements equals the number of elements covered by or fewer elements, and its extension, that the incidence matrix or combinatorial Radon transform between these two sets of elements is invertible.

The Möbius function has a homological interpretation. The value for a partially ordered set with minimum and maximum is the Euler characteristic of the order complex of , the simplicial complex whose simplices are the chains of , the partially ordered set with and deleted. Because polytopes are topologically spheres, this yields the following theorem. If and are faces in the face lattice of a polytope and , then , where is the difference . Taking the nerve of a covering of the order complex (cf. also Nerve of a family of sets), one obtains a homology based on a cross-cut and the cross-cut theorem.

The homological interpretation is especially interesting for a geometric lattice . In this case, the only non-trivial homology groups (cf. Homology group) are and , where is the rank of , and is the rank or dimension of the top homology group . Rota has proved the following sign theorem: If is a rank- flat in a geometric lattice, then is positive. Indeed, counts certain subsets in the broken-circuit complex defined by H. Whitney.

The characteristic polynomial of a ranked partially ordered set is the polynomial

in the variable . With simple modifications, one can obtain from the characteristic polynomial of a geometric lattice the Poincaré polynomial of an arrangement of hyperplanes and the chromatic polynomial of a graph (cf. also Graph colouring). The characteristic polynomial is an essential tool in the critical problem for matroids (cf. also Matroid). Related to the characteristic polynomial is the Eulerian function of a finite group , defined to be the Dirichlet polynomial

where the sum ranges over all subgroups in the subgroup lattice of . When is a positive integer, is the number of -tuples of group elements whose underlying set generate .

The Möbius invariant of a matroid is the integer with calculated in its lattice of flats if has no loops, and otherwise. Except when the element is an isthmus, satisfies the relation

where is with deleted and is contracted at . The triple is an analogue of a short exact sequence (cf. also Exact sequence). Many other invariants (including the characteristic polynomial) satisfy contraction-and-deletion relations. In addition, inductive arguments involving contractions and deletions are often used in proofs.

#### References

[a1] | M. Barnabei, A. Brini, G.-C. Rota, "Theory of Möbius functions" Russian Math. Surveys , 3 (1986) pp. 135–188 |

[a2] | A. Björner, "Homology and shellability of matroids and geometric lattices" N.L. White (ed.) , Matroid Applications , Cambridge Univ. Press (1992) pp. 226–283 |

[a3] | J.P.S. Kung, "Radon transforms in combinatorics and lattice theory" I. Rival (ed.) , Combinatorics and Ordered Sets , Amer. Math. Soc. (1986) pp. 33–74 |

[a4] | G.-C. Rota, "On the foundations of combinatorial theory I: Theory of Möbius functions" Z. Wahrsch. Verw. Gebiete , 2 (1964) pp. 340–368 |

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Möbius inversion.

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