Graph Colouring and Cartesian Closed Categories
Posted by Tom Leinster
Just for fun, here’s a reformulation of a famous conjecture in terms of the cartesian closed structure on the category of graphs.
Hedetniemi’s conjecture says there’s no clever way of colouring a product of graphs. By “colouring”, I mean colouring the vertices in such a way that the ends of each edge get different colours.
Given graphs and , any colouring of gives rise to a colouring of : just paint the same colour as . Of course, the same is true with and interchanged. Hedetniemi conjectured that if you want to colour with as few colours as possible, you can’t beat this method. That is, writing for the chromatic number of a graph — the smallest number of colours needed to colour it —
This has been open since 1966.
I’ll explain how Hedetniemi’s conjecture is equivalent to the following statement about the cartesian closed category of graphs: for any graph and complete graph , either or has a point.
For the purposes of this post, a graph is a finite set with a symmetric relation on it, and a homomorphism is a map of vertex-sets preserving the relation. Concretely, then, our graphs are undirected and can’t have multiple edges, but each vertex may have a loop on it (an edge from it to itself).
The category of graphs has finite products. They’re what you’d imagine: a vertex of is a pair , and vertices and are adjacent (that is, there’s an edge between them) if and only if is adjacent to and is adjacent to .
The terminal graph consists of a single vertex with a loop on it. In a category with a terminal object , a point or global element of an object is simply a map . Here, then, a point of a graph is a loop in .
Graph theorists most often like to consider “pointless graphs” — those without loops. But for what we’re doing here, it’s important to allow graphs to have points.
The category of graphs not only has finite products; it’s also cartesian closed. This means that for any graphs and , there is another graph with the following property: for all graphs , there is a natural one-to-one correspondence between homomorphisms
and homomorphisms
Here’s what looks like. A vertex is not a homomorphism from to , as you might guess; it’s any function . Two functions are adjacent in if and only if
In particular, is adjacent to itself in (meaning that there is a loop on ) if and only if is a homomorphism . Differently put, the points of are exactly the homomorphisms . This is the special case of the one-to-one correspondence just mentioned.
Now for colourings. As I wrote about once before, a (proper) colouring of a graph by colours is nothing more than a homomorphism from to the complete graph . By definition, has vertices and an edge between each pair of distinct vertices. It contains no loops, so it’s in some sense a maximal pointless graph.
I want to draw your attention to a trivial but important point, which I’ll give a grand name so that I can refer to it later:
The Exclusivity Principle Let be a graph and a complete graph. Then and cannot both have points.
Why? Well, a point of is a homomorphism , and a point of is a homomorphism , so if we have both then we can compose to get a point — and that’s impossible, as complete graphs are pointless. Like I said, trivial.
Another way to put this is that a graph with a point has no colourings. Concretely, this is simply the fact that if our graph contains a loop then we fall foul of the condition that different ends of each edge have to be different colours: they can’t be, because they’re the same vertex!
The chromatic number of a graph is the smallest number of colours you need to colour it:
A pointless graph can always be coloured by painting each vertex a different colour. On the other hand, we just saw that if a graph has any points at all, it has no colouring. So if and only if is pointless.
Any graph homomorphism induces a function from the set of -colourings of to those of , by composition. In particular, if is -colourable then so is . Let’s write if there exists a homomorphism . Then
In particular, take graphs and . Since we have projections and ,
Conjecture (Hedetniemi, version 1) This is an equality.
In the case where or has a point, it’s trivial. For suppose that has a point. Then we have a map , as well as the projection . This gives , and so
So the substance of the conjecture is about pointless, or irreflexive, graphs.
To get to the rephrasing of the conjecture mentioned in the introduction, I want to move the focus away from the chromatic number and towards the family of complete graphs. So here’s a first, mild, rephrasing.
Conjecture (Hedetniemi, version 2) Let and be graphs and a complete graph. If there is a homomorphism , then there is a homomorphism or a homomorphism .
Writing as , this just says: if then or . That is, if then . That is, . Since the converse inequality is trivial, that’s equivalent to version 1.
Now here’s a third, slightly different-looking, version of Hedetniemi’s conjecture. I learned it from Godsil and Royle’s book Algebraic Graph Theory (section 6.6).
Conjecture (Hedetniemi, version 3) For any graph and complete graph , either or has a point.
The Exclusivity Principle implies that they can’t both have a point: so it’s a strict either/or.
Let’s prove that this is equivalent to version 2.
First suppose that version 3 is true. Take graphs and , and a complete graph . We’ll prove the contrapositive of version 2: if there’s no homomorphism and no homomorphism then there is no homomorphism . Supposing that there are no homomorphisms or , version 3 implies that there are homomorphisms and . So we get homomorphisms
The composite is a homomorphism . So by the Exclusivity Principle applied to , there’s no homomorphism .
Conversely, suppose that version 2 is true. Take a graph and a complete graph . The cartesian closed structure gives us a counit homomorphism
(which concretely is just ). Version 2 then tells us that either there’s a homomorphism or there’s a homomorphism . And that’s exactly what version 3 says.
So that’s the rephrasing of Hedetniemi’s conjecture. As I said, it’s just for fun: there’s no evidence that it leads anywhere.
You could also state a fourth version:
Conjecture (Hedetniemi, version 4) For any graph and complete graph , at least one of the graphs has a point.
But that seems like just a dressing-up of version 3. Why? Well, for any graph , the cartesian closed structure gives us a unit map . Taking and in turn gives us maps
(Category theorists will now mumble the words “triangle identity”.) So has a point iff does. More generally, by the same argument, the th member of this sequence has a point iff the th member does, for . So we might as well truncate the sequence to
We can also drop the , since the unit map tells us that if has a point then does too. So we’re back to version 3.
Re: Graph Colouring and Cartesian Closed Categories
If you could use the ideas to help prove the Hedetniemi conjecture, category theorists would be eternally grateful, since they’d have a nice answer to “what has category theory ever done for graph theory?”
Of course you’ve already thought of this.