A modular lattice is a lattice where “opposite sides” of a “diamond” formed by four points , , , are “congruent”.
A modular lattice is a lattice which satisfies a modular law, which we introduce after a few preliminaries.
where is left adjoint to . Indeed, for any , we have a unit
whereas for any , we have dually a counit
A lattice is modular if for any , the adjoint pair
is an adjoint equivalence.
This is perhaps the most memorable definition for a category theorist: it is a precise expression of the slogan given in the Idea section.
It is immediate that the concept of modular lattice is self-dual, i.e., if is modular, then so is .
In the lattice-theoretic literature, modularity is usually formulated somewhat differently. Here are three alternative conditions on a lattice, all equivalent to that of Definition 1.
The modular law is the universal Horn sentence
The modular identity is the universal equation
“Freyd’s modular law” (for lack of better term; see allegory) is the universal inequality
To see that the modular identity follows from Definition 1, observe that for any we have
Let . Under , this element is sent to
Under Definition 1, this last element is sent back to by . Therefore we have
and since this is true for all , we can interchange and and rearrange by commutativity to get
which is the modular identity.
To get the modular law from the modular identity, just use the fact that the hypothesis is equivalent to , and use this to substitute for in the modular identity. Conversely, from the tautology , we can instantiate the modular law to derive the modular identity.
From the tautology , it is clear that Freyd’s modular law follows from the modular identity. Conversely, by substituting for in Freyd’s modular law, we derive the special case
whereas the opposite inequality
holds in any lattice, so the modular identity follows from Freyd’s modular law.
Finally, we derive the adjoint equivalence of Definition 1 from the modular identity. One half of the adjoint equivalence states that if , then ; if this holds, then the other half follows because it is the dual statement. If , then
just by the laws of a lattice. By the modular identity (again switching and ), the right side equals . But since , this equals , as was to be shown.
which proves the modular law.
For any Mal'cev variety or Mal’cev algebraic theory, the lattice of internal equivalence relations of an algebra is a modular lattice. The equivalence classes often arise as cosets of kernels; for example, for a vector space , equivalence relations correspond to subspaces of , and form a modular lattice. Other examples include the lattice of normal subgroups of a group, the lattice of two-sided ideals of a ring, etc.
Every abstract projective plane gives rise to a modular lattice whose underlying set is the disjoint union
where is taken as bottom, as top, the points are atoms, and the lines are coatoms, ordered by the incidence relation. The projective plane need not be Desarguesian.
The smallest non-modular lattice has 5 elements and is called the pentagon, denoted . It can be described as the lattice where and is incomparable with and .
(Dedekind) A lattice is modular if and only if there is no injective function that preserves meets and joins.
(Notice we are leaving out the condition of preservation of the top and bottom elements.) The direction is easy enough: being modular is incompatible with an injective preserving meets and joins, since we contradict the modular law in by applying to and .
This is reminiscent of forbidden minor characterizations of certain classes of graphs; see graph minor. There is a similar “forbidden sublattice” characterization of distributive lattices – see this comment by Tom Leinster at the -Category Café.
Free modular lattices tend to be complicated. Dedekind showed that the free modular lattice on 3 elements has 28 elements; its Hasse diagram can be seen in these lecture notes by J.B. Nation (chapter 9, page 100).
N.B.: this notion of lattice is meant with respect to the signature ; if we include top and bottom constants in the signature, then the free modular lattice on three elements has 30 elements. A compelling illustration (in gif form) which exhibits triality of this lattice is given in this -Category Café post, as part of a larger discussion which explores the connection with linear representations of the quiver (the Coxeter diagram of ).
For , the free modular lattice generated by elements is infinite and in fact has an undecidable word problem (Freese, Herrmann).
C. Herrmann, On the word problem for the modular lattice with four free generators, Mathematische Annalen 265 (1983), 513-527. (Springerlink)
J.B. Nation, Revised Notes on Lattice Theory. Available here: (web)
Tom Leinster, Comment on Solèr’s Theorem, December 4, 2010. (link)