Modular lattice

Hasse diagram of N₅, the smallest non-modular lattice.

In the branch of mathematics called order theory, a modular lattice is a lattice that satisfies the following self-dual condition:

Modular law

x ≤ b implies x ∨ (a ∧ b) = (x ∨ a) ∧ b,

where  ≤  is the partial order, and  ∨  and  ∧ (called join and meet respectively) are the operations of the lattice. Modular lattices arise naturally in algebra and in many other areas of mathematics. For example, the subspaces of a vector space (and more generally the submodules of a module over a ring) form a modular lattice.

Every distributive lattice is modular.

In a not necessarily modular lattice, there may still be elements b for which the modular law holds in connection with arbitrary elements a and x (≤ b). Such an element is called a modular element. Even more generally, the modular law may hold for a fixed pair (a, b). Such a pair is called a modular pair, and there are various generalizations of modularity related to this notion and to semimodularity.

Introduction

The modular law can be seen (and memorized) as a restricted associative law that connects the two lattice operations similarly to the way in which the associative law λ(μx) = (λμ)x for vector spaces connects multiplication in the field and scalar multiplication. The restriction x ≤ b is clearly necessary, since it follows from x ∨ (a ∧ b) = (x ∨ a) ∧ b.

Nonmodularity of N₅.

It is easy to see that x ≤ b implies x ∨ (a ∧ b) ≤ (x ∨ a) ∧ b in every lattice. Therefore the modular law can also be stated as

Modular law (variant)

x ≤ b implies x ∨ (a ∧ b) ≥ (x ∨ a) ∧ b.

By substituting x with x ∧ b, the modular law can be expressed as an equation that is required to hold unconditionally, as follows:

Modular identity

(x ∧ b) ∨ (a ∧ b) = [(x ∧ b) ∨ a] ∧ b.

This shows that, using terminology from universal algebra, the modular lattices form a subvariety of the variety of lattices. Therefore all homomorphic images, sublattices and direct products of modular lattices are again modular.

The smallest non-modular lattice is the "pentagon" lattice N₅ consisting of five elements 0,1,x,a,b such that 0 < x < b < 1, 0 < a < 1, and a is not comparable to x or to b. For this lattice x ∨ (a ∧ b) = x ∨ 0 = x < b = 1 ∧ b = (x ∨ a) ∧ b holds, contradicting the modular law. Every non-modular lattice contains a copy of N₅ as a sublattice.

Modular lattices are sometimes called Dedekind lattices after Richard Dedekind, who discovered the modular identity.

Diamond isomorphism theorem

For any two elements a,b of a modular lattice, one can consider the intervals [a ∧ b, b] and [a, a ∨ b]. They are connected by order-preserving maps

φ: [a ∧ b, b] → [a, a ∨ b] and

ψ: [a, a ∨ b] → [a ∧ b, b]

that are defined by φ(x) = x ∨ a and ψ(x) = x ∧ b.

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  In a modular lattice, the maps φ and ψ indicated by the arrows are mutually inverse isomorphisms.

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  Failure of the diamond isomorphism theorem in a non-modular lattice.

The composition ψφ is an order-preserving map from the interval [a ∧ b, b] to itself which also satisfies the inequality ψ(φ(x)) = (x ∨ a) ∧ b ≥ x. The example shows that this inequality can be strict in general. In a modular lattice, however, equality holds. Since the dual of a modular lattice is again modular, φψ is also the identity on [a, a ∨ b], and therefore the two maps φ and ψ are isomorphisms between these two intervals. This result is sometimes called the diamond isomorphism theorem for modular lattices. A lattice is modular if and only if the diamond isomorphism theorem holds for every pair of elements.

The diamond isomorphism theorem for modular lattices is analogous to the third isomorphism theorem in algebra, and it is a generalization of the lattice theorem.

Modular pairs and related notions

The centred hexagon lattice S₇, also known as D₂, is M-symmetric but not modular.

In any lattice, a modular pair is a pair (a, b) of elements such that for all x satisfying a ∧ b ≤ x ≤ b, we have (x ∨ a) ∧ b = x, i.e. if one half of the diamond isomorphism theorem holds for the pair.^[1] An element b of a lattice is called a (right) modular element if (a, b) is a modular pair for all elements a.

A lattice with the property that if (a, b) is a modular pair, then (b, a) is also a modular pair is called an M-symmetric lattice.^[2] Since a lattice is modular if and only if all pairs of elements are modular, clearly every modular lattice is M-symmetric. In the lattice N₅ described above, the pair (b, a) is modular, but the pair (a, b) is not. Therefore N₅ is not M-symmetric. The centred hexagon lattice S₇ is M-symmetric but not modular. Since N₅ is a sublattice of S₇, it follows that the M-symmetric lattices do not form a subvariety of the variety of lattices.

M-symmetry is not a self-dual notion. A dual modular pair is a pair which is modular in the dual lattice, and a lattice is called dually M-symmetric or M^*-symmetric if its dual is M-symmetric. It can be shown that a finite lattice is modular if and only if it is M-symmetric and M^*-symmetric. The same equivalence holds for infinite lattices which satisfy the ascending chain condition (or the descending chain condition).

Several less important notions are also closely related. A lattice is cross-symmetric if for every modular pair (a, b) the pair (b, a) is dually modular. Cross-symmetry implies M-symmetry but not M^*-symmetry. Therefore cross-symmetry is not equivalent to dual cross-symmetry. A lattice with a least element 0 is ⊥-symmetric if for every modular pair (a, b) satisfying a ∧ b = 0 the pair (b, a) is also modular.

History

The definition of modularity is due to Richard Dedekind, who published most of the relevant papers after his retirement. In a paper published in 1894 he studied lattices, which he called dual groups (German: Dualgruppen) as part of his "algebra of modules" and observed that ideals satisfy what we now call the modular law. He also observed that for lattices in general, the modular law is equivalent to its dual.

In another paper in 1897, Dedekind studied the lattice of divisors with gcd and lcm as operations, so that the lattice order is given by divisibility. In a digression he introduced and studied lattices formally in a general context. He observed that the lattice of submodules of a module satisfies the modular identity. He called such lattices dual groups of module type (German: Dualgruppen vom Modultypus). He also proved that the modular identity and its dual are equivalent.

In the same paper, Dedekind observed further that any lattice of ideals of a ring satisfies the following stronger form of the modular identity, which is also self-dual:

(x ∧ b) ∨ (a ∧ b) = [x ∨ a] ∧ b.

He called lattices that satisfy this identity dual groups of ideal type (German: Dualgruppen vom Idealtypus). In modern literature, they are more commonly referred to as distributive lattices. He gave examples of a lattice that is not modular and of a modular lattice that is not of ideal type.

A paper published by Dedekind in 1900 had lattices as its central topic: He described the free modular lattice generated by three elements, a lattice with 28 elements.

See also

• Young–Fibonacci lattice, an infinite modular lattice defined on strings of the digits 1 and 2.
• Orthomodular lattice

Notes

1. ^ The French term for modular pair is couple modulaire. A pair (a, b) is called a paire modulaire in French if both (a, b) and (b, a) are modular pairs.
2. ^ Some authors, e.g. Fofanova (2001), refer to such lattices as semimodular lattices. Since every M-symmetric lattice is semimodular and the converse holds for lattices of finite length, this can only lead to confusion for infinite lattices.

References

• Corry, Leo, Modern algebra and the rise of mathematical structures (2nd ed.), pp. 121–129, ISBN 978-3764370022 
• Dedekind, Richard (1897), "Ueber Zerlegungen von Zahlen durch ihre grössten gemeinsamen Teiler", Braunschweiger Festschrift: 1–40 
• Dedekind, Richard (1900), "Ueber die von drei Moduln erzeugte Dualgruppe", Mathematische Annalen 53 (3): 371–403, doi:10.1007/BF01448979, http://resolver.sub.uni-goettingen.de/purl?GDZPPN002257947 
• Fofanova, T. S. (2001), "Semi-modular lattice", in Hazewinkel, Michiel, Encyclopaedia of Mathematics, Springer, ISBN 978-1556080104, http://eom.springer.de/s/s084240.htm 
• Maeda, Shûichirô (1965), "On the symmetry of the modular relation in atomic lattices", Journal of Science of the Hiroshima University 29: 165–170, http://projecteuclid.org/DPubS?verb=Display&version=1.0&service=UI&handle=euclid.hmj/1206139232&page=record 
• Rota, Gian-Carlo (1997), "The many lives of lattice theory", Notices of the American Mathematical Society 44 (11): 1440–1445, ISSN 0002-9920, http://www.ams.org/notices/199711/comm-rota.pdf 
• Skornyakov, L. A. (2001), "Modular lattice", in Hazewinkel, Michiel, Encyclopaedia of Mathematics, Springer, ISBN 978-1556080104, http://eom.springer.de/m/m064460.htm 
• Stern, Manfred (1999), Semimodular lattices, Cambridge University Press, ISBN 978-0-521-46105-4 

External links

• Modular lattice on PlanetMath