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When we do computations* in abstract commutative rings, we use a mental model all the time: the arithmetic of integers. After the foundations are developed, we don't really care about the specific ring axioms, especially when handling complex expressions. For abstract boolean algebras, we think of propositional logic and truth values. For fields, the mental model will usually be the rational or real numbers. For abelian categories, we usually think of the category of modules over a ring. For partial orders, already the choice of the symbol $\leq$ indicates that we usually think of the common less-than-or-equal relation of numbers.

Now what about lattices? Which mental model do most professionals** use for computations in abstract lattices?

More formally perhaps: What is a "small" class of "concrete" lattices ("models") such that an equation in the language of lattices holds in all models if and only if it holds in all lattices? The word "concrete" shall exclucde free lattices, which otherwise would provide a trivial answer.

For the purpose of this question, all lattices will be bounded (have smallest and largest element). I ask this because even though I can do computations in lattices, right now they are a bit "mechanic". Every step has to be done by hand, using the lattice axioms. This is much more tedious than computations in, say, rings, where I can just rely on my experience with integers.

For distributive lattices we can always use the mental model of sets with $\vee$ = union, $\wedge$ = intersection. Formally, this is justified by Birkhoff's representation theorem. (This theorem holds for every finite distributive lattice, but every valid expression lies in a finite distributive sublattice, so that is sufficient.)

Unfortunately, since not every lattice is distributive, this means that sets with $\vee$ = union, $\wedge$ = intersection are not an adequate mental model for lattices. They are not sufficient to distinguish the expressions $x \wedge (y \vee z)$ and $(x \wedge y) \vee (x \wedge z)$.

The prime example of a modular lattice is the lattice of submodules of a module, and I believe this was also the reason for the terminology. However, from the mathoverflow question Representation theorem for modular lattices (a question I completely forgot!) the lattice of submodules satisfies further identities, such as the Arguesian law. This indicates that submodules of a module are probably not a good mental model for modular lattices.

I know that a lattice is modular (resp., distributive) iff it does not contain a copy of $N_5$ (resp., $M_3$ or $N_5$), but I am afraid that this doesn't help with computations.

I saw here and in some papers that the notation $ab = a \wedge b$ and $a+b = a \vee b$ is used. However, this means that I need to become comfortable with identities such as $a + a = a$ and $a + ab = a$, which is a bit weird since I have to "override" my mental model for the rules of $+$ and $\cdot$.

*With computations, I mean specifcally mean deductions in equational logic. So, even though the ring of integers has all sorts of special properties (principal ideal domain, characteristic $0$, ...), the ring-theoretic equations satisfied by $\mathbb{Z}$ are only those satisfied in every commutative ring. This is why $\mathbb{Z}$ is an adequate model for general commutative rings. Formally: if $p,q \in \mathbb{Z}[T_1,\dotsc,T_n]$ are two polynomials such that $p(z_1,\dotsc,z_n) = q(z_1,\dotsc,z_n)$ for all $z_1,\dotsc,z_n \in \mathbb{Z}$, then $p = q$ as polynomials.

**I have formulated it this way to prevent this question from being opinion-based and then perhaps getting closed. While I am looking forward to hearing about your personal preferences for mental models in the comments, I am looking for answers preferably from mathematicians who have professionally worked with lattices before and therefore know what is "standard" in the field.

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    Wikipedia suggests a canonical mental model of the powerset of a set (which I think is exemplified in your "distributive lattice" example). I'm not too familiar with lattices but perhaps that might be a useful starting point. This next part is a half baked idea but "LCMs" and "GCDs" over a ring seem like they encapsulate the lattice concept as well. There might be a useful mental model that can be made there. – Sidharth Ghoshal Oct 15 '24 at 18:35
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    I have just added a paragraph explaining why that mental model is not adequate for lattices (simply: not every lattice is distributive). – Martin Brandenburg Oct 15 '24 at 18:35
  • I think: What the integers are for ring theory, the power set is for lattice theory. Both are the prototypical object of their kind, whose properties are the basis for defining special classes (Euclidean / principal ideal / UFD / domain on the ring side; distributive / modular / unique complements etc. on the lattice type). Concerning "not every lattice is distributive", I mean, also not every ring is a UFD. – azimut Oct 15 '24 at 19:44
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    I don't agree with that comparison. The equational theory of the integers is the same as that of general commutative rings. But the equational theory of the power set lattice is not of a general lattice. Notice that equational logic is very restricted, it basically only talks about terms that are equal for all variables. So being an UFD is not covered by that. For doing elementary computations, the equational theory seems to be most relevant. – Martin Brandenburg Oct 15 '24 at 19:46
  • @azimut I disagree with that statement. The variety generated by the powerset lattices is much smaller than the variety of lattices itself, so powerset lattices actually do a pretty bad job of modeling "generic" lattices. – Noah Schweber Oct 15 '24 at 20:00
  • @NoahSchweber I did not say that powersets do model "generic" lattices. Neither do the integers model "generic" rings. – azimut Oct 15 '24 at 20:10
  • @MartinBrandenburg Maybe you should add the "equational theory" point more explicitly to the equation. Now that I got your point, I agree that being able to compute in distributive lattices doesn't help much for computations in generic modular lattices (but being able to compute in the integers helps for generic rings). Interesting question! – azimut Oct 15 '24 at 20:14
  • @azimut Thank you for the feedback. I have made a few improvements. – Martin Brandenburg Oct 15 '24 at 22:06

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Here's a partial answer. Every algebraic lattice is isomorphic to the congruence lattice of some algebra (Gratzer/Schmidt). In particular, every finite lattice is isomorphic to the congruence lattice of some algebra, and if memory serves the equational theory of algebraic lattices is no larger than the equational theory of lattices itself. So at least in a wide variety of circumstances, we can get away with thinking of a lattice as a lattice of congruences.

Noah Schweber
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    Memory serves right: for example, in left-zero semigroups (among others), every equivalence relation is a congruence. So their congruence lattices are all partition lattices; and the class of partition lattices doesn't have any additional identity, for every lattice is embeddable into one of these. – amrsa Oct 15 '24 at 20:04
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The quantum logic program (see the wiki page) aimed at proving that any orthomodular orthocomplemented lattice was a sublattice of the lattice of closed subspaces in a (generalized) Hilbert space, but unfortunately, there seems to be no conclusive proof.

Plop
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For modular lattices this is too much to ask, since the word problem is unsolvable even for free modular lattices. Though you can get pretty far if you just think about modules. It would be hard to stumble across non-Desarguesian planes if you didn't know about them already.

For general lattices, Noah's suggestion is a good one, and universal algebraicists contribute to general lattice theory pretty regularly. If you don't want to picture a whole algebra, every lattice can be represented as a lattice of equivalence relations that are 4-permutable. Modular lattices can be represented as 3-permutable equivalence relations. (You can find the precise result here: https://math.hawaii.edu/~jb/math618/os4uh.pdf)

Another formulation is that can also embed every lattice in the lattice of closed sets for some closure operator. (The algebraic lattice result can be thought as a strengthening of this to imposing a restriction on the type of closure operator.)

The word problem for free lattices is solvable, so you could also look at that directly. Freese, Jezek and Nation have a book on free lattices that give the result.

arsmath
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