# Poisonous Shapes in Algebra and Graph Theory

I recently started learning about Algebraic Lattices because I was interested in how they relate to Automata Theory. It turns out they’re fundamental to a bunch of other things that I’ve always wanted to learn more about, like CRDTs and Type Theory.

I stumbled upon an interesting property about lattices that reminded me of something from Graph Theory…

## Distributive Lattices

First, let’s start with the mathy definition of a lattice given by Wikipedia:

If $(L, \leq)$ is a partially ordered set, and $S \subseteq L$ is an arbitrary subset, then an element $u \in L$ is said to be an upper bound of $S$ if $s \leq u$ for each $s \in S$. A set may have many upper bounds, or none at all. An upper bound $u$ of $S$ is said to be its least upper bound, or join, or supremum, if $u \leq x$ for each upper bound $x$ of $S$. A set need not have a least upper bound, but it cannot have more than one. Dually, $l \in L$ is said to be a lower bound of $S$ if $l \leq s$ for each $s \in S$. A lower bound $l$ of $S$ is said to be its greatest lower bound, or meet, or infimum, if $x \leq l$ for each lower bound $x$ of $S$. A set may have many lower bounds, or none at all, but can have at most one greatest lower bound.

A partially ordered set $(L, \leq)$ is called a join-semilattice and a meet-semilattice if each two-element subset $\{a,b\} \subseteq L$ has a join (i.e. least upper bound) and a meet (i.e. greatest lower bound), denoted by $a \vee b$ and $a \wedge b$, respectively. $(L, \leq)$ is called a lattice if it is both a join- and a meet-semilattice.

That’s a lot to take in, but it’s not so bad once you start looking at lattices using Hasse diagrams. A Hasse diagram is a visualization of a partially ordered set, where each element is drawn below the other elements that it is less than. Edges are drawn between two elements $x$ and $y$ if $x < y$ and there is no $z$ such that $x < z < y$.

For example, if we define the $\leq$ operator to be “is a subset of”, we get a partial ordering for any powerset. Here’s the Hasse diagram for the lattice $(\mathcal{P}(\{1,2,3\}), \subseteq)$:

In this case, the $\subseteq$ operator results in $\vee$ (the “join”) being union of the sets and $\wedge$ (the “meet”) as the intersection of the sets. It’s a tragic failure of notation that $\vee$ means you should find the common connected element that is above the pair in the diagram and $\wedge$ means you should look below.

Another example is the set of divisors of a number, where the $\leq$ operator means “divides into”. Here’s the Hasse diagram for all the divisors of 30:

In this case, $\vee$ ends up meaning Least Common Multiple and $\wedge$ means Greatest Common Divisor.

Yet another example is the set of binary strings of a fixed length $n$, with $S \leq T$ meaning $S_i \leq T_i$ for $1 \leq i \leq n$:

In this case, $\vee$ is the binary OR operator and $\wedge$ is the binary AND operator.

These examples are interesting because they’re all the same structure! If we can prove a fact about the structure, we get information about all three systems. These happen to be Boolean lattices, which have the distributive property:

$$a \vee (b \wedge c) = (a \vee b) \wedge (a \vee c)$$ $$\forall a, b, c \in L$$

Now here’s the neat part. There are fundamentally only two lattices that aren’t distributive:

The Diamond Lattice

$$a \vee (b \wedge c) \stackrel{?}{=} (a \vee b) \wedge (a \vee c)$$ $$a \vee 0 \stackrel{?}{=} 1 \wedge 1$$ $$a \neq 1$$ …and:

The Pentagon Lattice

$$a \vee (b \wedge c) \stackrel{?}{=} (a \vee b) \wedge (a \vee c)$$ $$a \vee 0 \stackrel{?}{=} 1 \wedge c$$ $$a \neq c$$

Any lattice that has a sub-lattice that is isomorphic to either the Diamond or Pentagon, referred to as $M_3$ and $N_5$ respectively, is not distributive. It’s not too hard to see, just verify that the corresponding elements that violate distribution above will fail no matter what else is going on in the lattice.

The surprising part is that every non-distributive lattice must contain either $M_3$ or $N_5$! In a sense these are the two poisonous minimal shapes: their presence kills distributivity.

This alone seems like a cool little fact, but what struck me was that this isn’t the first time that I’ve seen something like this…

## Planar Graphs

In Graph Theory, there’s a property of graphs called planarity. If you can draw the graph without any of the edges crossing, the graph is planar. The Wikipedia page on planar graphs is filled with awesome facts. For example, a planar graph can’t have an average degree greater than 6 and any graph that can be drawn planarly with curvy edges can be drawn planarly with straight edges, but the most famous result concerning planar graphs is Kuratowski’s Theorem:

A finite graph is planar if and only if it does not contain a subgraph that is a subdivision of $K_5$ (the complete graph on five vertices) or of $K_{3,3}$ (the complete bipartite graph on six vertices)

$K_5$, the complete graph on five vertices

$K_{3,3}$, the complete bipartite graph on six vertices

It turns out there are fundamentally only two graphs that aren’t planar! Any graph that “contains” either of these two graphs is not planar and any graph that isn’t planar must “contain” one of them. The meaning of “contains” is a little different here. It’s not enough to just find vertices that form $K_5$ or $K_{3,3}$ with the edges that connect them. What we need to check is if any subgraph can be “smoothed” into one of these two graphs.

For example, this graph isn’t planar because the red vertex can be smoothed away, leaving $K_{3,3}$:

## And?

Unfortunately, I’m just learning about lattice theory and I have no idea if there’s any formal connection between these similar concepts. I have found that there is a branch of lattice theory that is concerned with the planarity of the Hasse diagrams, so it’s not like any algebraist hasn’t thought of this before. If anyone has a formal connection or an example of other cases where a pair of structures have this kind of “poisonous” property, I’d love to hear.

## Addendum

Nerkbot over at /r/puremathematics was kind enough to share:

You might be interested in the Robertson-Seymour theorem which says that for any graph property that is closed under taking minors there is a finite list of “forbidden minors” or as you call them “poisonous” graphs. Kuratowski’s theorem is one particular example of this.

This relates also to the concept of well-quasi-orders. A quasi-ordered set $(X, \leq)$ is a well-quasi-order if every infinite sequence ($x_1, x_2, …$) with elements in $X$ has some $i < j$ with $x_i \leq x_j$. An equivalent statement of the Robertson-Seymour theorem is that the set of all finite graphs ordered by the minor relation is well-quasi-ordered.

Another example where “forbidden minors” appear is matroids. Matroids also have minors but unlike graphs they are not well-quasi-ordered under the minor relation. However certain minor closed properties do have a finite list of forbidden minors. For example Rota’s conjecture, which was announced to be proven very recently, says that for any finite field the set of matroids that are representable over that field are characterized by a finite list of forbidden minors.