Semimodular lattice

In the branch of mathematics known as order theory, a semimodular lattice, is a lattice that satisfies the following condition:

semimodular implication: a ∧ b <: a implies b <: a ∨ b.

The notation a <: b means that b covers a, i.e. a < b and there is no element c such that a < c < b.

An algebraic atomistic semimodular bounded lattice is called a matroid lattice because such lattices are equivalent to matroids. An atomistic semimodular bounded lattice of finite length is called a geometric lattice and corresponds to a matroid of finite rank.^[1]

Semimodular lattices are also known as upper semimodular lattices; the dual notion is that of a lower semimodular lattice. A finite lattice is modular if and only if it is both upper and lower semimodular.

A finite lattice, or more generally a lattice satisfying the ascending chain condition or the descending chain condition, is semimodular if and only if it is M-symmetric. Some authors refer to M-symmetric lattices as semimodular lattices.^[2]

Birkhoff's condition

A lattice is sometimes called weakly semimodular if it satisfies the following condition due to Garrett Birkhoff:

Birkhoff's condition: a ∧ b <: a and a ∧ b <: b implies a <: a ∨ b and a <: a ∨ b.

Every semimodular lattice is weakly semimodular. The converse is true for lattices of finite length, and more generally for upper continuous strongly atomic lattices.

Mac Lane's condition

The following two conditions due to Saunders Mac Lane are equivalent:

Mac Lane's condition 1: For any a, b, c such that b ∧ c < a < c < b ∨ a,; there is an element d such that b ∧ c < d ≤ b and a = (a ∨ d) ∧ c.
Mac Lane's condition 1: For any a, b, c such that b ∧ c < a < c < b ∨ c,; there is an element d such that b ∧ c < d ≤ b and a = (a ∨ d) ∧ c.

Every lattic satisfying Mac Lane's condition is semimodular. The converse is true for lattices of finite length, and more generally for strongly atomic lattices. Moreover, every upper continuous lattice satisfying Mac Lane's condition is M-symmetric.

References

^ These definitions follow Stern (1999). Some authors use the term geometric lattice for the more general matroid lattices.
^ For instance Fofanova (2001).

External links

"Geometric lattice". PlanetMath.. (The article is about matroid lattices.)
"Semimodular lattice". PlanetMath..

References

Fofanova, T. S. (2001) [1994], "Semi-modular lattice", Encyclopedia of Mathematics, EMS Press. (The article is about M-symmetric lattices.)
Stern, Manfred (1999), Semimodular lattices, Cambridge University Press, ISBN 978-0-521-46105-4.

[1] These definitions follow Stern (1999). Some authors use the term geometric lattice for the more general matroid lattices.

[2] For instance Fofanova (2001).

[1]

[2]