This is about a notion in order theory/logic. For an unrelated notion of a similar name in group theory/quadratic form-theory see at modular integral lattice.
A modular lattice is a lattice where “opposite sides” of a “diamond” formed by four points $x \wedge y$, $x$, $y$, $x \vee y$ are “congruent”.
A modular lattice is a lattice which satisfies a modular law, which we introduce after a few preliminaries.
In any lattice $L$, given two elements $x, y \in L$ with $x \leq y$, let $[x, y]$ denote the interval $\{z \colon x \leq z \leq y\}$. Then, given any two elements $a, b \in L$, there is an adjoint pair
where $a \vee (-)$ is left adjoint to $(-) \wedge b$. Indeed, for any $w \in [a \wedge b, b]$, we have a unit
whereas for any $z \in [a, a \vee b]$, we have dually a counit
A lattice $L$ is modular if for any $a, b \in L$, 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 $L$ is modular, then so is $L^{op}$.
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 $z \in L$ we have
Let $w = (a \vee z) \wedge (a \vee b)$. Under $(-) \wedge b \colon [a, a \vee b] \to [a \wedge b, b]$, this element $w$ is sent to
Under Definition 1, this last element is sent back to $w$ by $a \vee (-)$. Therefore we have
and since this is true for all $a, b, z$, we can interchange $z$ and $b$ 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 $a \leq b$ is equivalent to $a \vee b = b$, and use this to substitute $b$ for $a \vee b$ in the modular identity. Conversely, from the tautology $a \leq a \vee b$, we can instantiate the modular law to derive the modular identity.
From the tautology $(a \vee z) \wedge b \leq (a \vee z) \wedge (a \vee b)$, it is clear that Freyd’s modular law follows from the modular identity. Conversely, by substituting $a \vee b$ for $b$ 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 $a \leq z \leq a \vee b$, then $z = a \vee (z \wedge b)$; if this holds, then the other half follows because it is the dual statement. If $a \leq z \leq a \vee b$, then
just by the laws of a lattice. By the modular identity (again switching $b$ and $z$), the right side equals $a \vee (b \wedge (a \vee z))$. But since $a \vee z = z$, this equals $a \vee (b \wedge z) = a \vee (z \wedge b)$, as was to be shown.
Every distributive lattice, e.g., a Heyting algebra, is modular. Indeed, if $a \leq b$ in a distributive lattice, we have
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 $V$, equivalence relations correspond to subspaces of $V$, 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.
In fact, any lattice of commuting equivalence relations on a set is a modular lattice (being a suballegory of the allegory of sets, one in which composition provides the join).
Every abstract projective plane gives rise to a modular lattice $L$ whose underlying set is the disjoint union
where $0$ is taken as bottom, $1$ 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 $N_5$. It can be described as the lattice $\{\bot, a, b, c, \top\}$ where $b \leq c$ and $a$ is incomparable with $b$ and $c$.
A lattice $L$ is modular if and only if there is no injective function $f \colon N_5 \to L$ that preserves meets and joins.
(Notice we are leaving out the condition of preservation of the top and bottom elements.)
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 $n$-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).
For $n \geq 4$, the free modular lattice generated by $n$ 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)