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December 5, 2014

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 XX and YY, any colouring of XX gives rise to a colouring of X×YX \times Y: just paint (x,y)(x, y) the same colour as xx. Of course, the same is true with XX and YY interchanged. Hedetniemi conjectured that if you want to colour X×YX \times Y with as few colours as possible, you can’t beat this method. That is, writing χ\chi for the chromatic number of a graph — the smallest number of colours needed to colour it —

χ(X×Y)=min{χ(X),χ(Y)}. \chi(X \times Y) = \min \{ \chi(X), \chi(Y) \}.

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 XX and complete graph KK, either K XK^X or K K XK^{K^X} has a point.

For the purposes of this post, a graph XX is a finite set V(X)V(X) with a symmetric relation E(X)E(X) 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 X×YX \times Y is a pair (x,y)V(X)×V(Y)(x, y) \in V(X) \times V(Y), and vertices (x,y)(x, y) and (x,y)(x', y') are adjacent (that is, there’s an edge between them) if and only if xx is adjacent to xx' and yy is adjacent to yy'.

The terminal graph 11 consists of a single vertex with a loop on it. In a category with a terminal object 11, a point or global element of an object XX is simply a map 1X1 \to X. Here, then, a point of a graph XX is a loop in XX.

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 YY and ZZ, there is another graph Z YZ^Y with the following property: for all graphs XX, there is a natural one-to-one correspondence between homomorphisms

XZ Y X \to Z^Y

and homomorphisms

X×YZ. X \times Y \to Z.

Here’s what Z YZ^Y looks like. A vertex is not a homomorphism from YY to ZZ, as you might guess; it’s any function V(Y)V(Z)V(Y) \to V(Z). Two functions θ,θ:V(Y)V(Z)\theta, \theta': V(Y) \to V(Z) are adjacent in Z YZ^Y if and only if

(y,y) adjacent in Y(θ(y),θ(y)) adjacent in Z. (y, y') \,\,\text{ adjacent in }\,\, Y \implies (\theta(y), \theta'(y')) \,\,\text{ adjacent in }\,\, Z.

In particular, θ:V(Y)V(Z)\theta: V(Y) \to V(Z) is adjacent to itself in Z YZ^Y (meaning that there is a loop on θ\theta) if and only if θ\theta is a homomorphism YZY \to Z. Differently put, the points of Z YZ^Y are exactly the homomorphisms YZY \to Z. This is the special case X=1X = 1 of the one-to-one correspondence just mentioned.

Now for colourings. As I wrote about once before, a (proper) colouring of a graph XX by nn colours is nothing more than a homomorphism from XX to the complete graph K nK_n. By definition, K nK_n has nn 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 XX be a graph and KK a complete graph. Then XX and K XK^X cannot both have points.

Why? Well, a point of XX is a homomorphism 1X1 \to X, and a point of K XK^X is a homomorphism XKX \to K, so if we have both then we can compose to get a point 1K1 \to K — 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:

χ(X)=min{n:Hom(X,K n)}{}. \chi(X) = \min \{ n : Hom(X, K_n) \neq \emptyset \} \in \mathbb{N} \cup \{\infty\}.

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 χ(X)<\chi(X) \lt \infty if and only if XX is pointless.

Any graph homomorphism WXW \to X induces a function Hom(X,K n)Hom(W,K n)\Hom(X, K_n) \to \Hom(W, K_n) from the set of nn-colourings of XX to those of WW, by composition. In particular, if XX is nn-colourable then so is WW. Let’s write WXW \leq X if there exists a homomorphism WXW \to X. Then

WXχ(W)χ(X). W \leq X \implies \chi(W) \leq \chi(X).

In particular, take graphs XX and YY. Since we have projections X×YXX \times Y \to X and X×YYX \times Y \to Y,

χ(X×Y)min{χ(X),χ(Y)}. \chi(X \times Y) \leq \min \{ \chi(X), \chi(Y) \}.

Conjecture (Hedetniemi, version 1)   This is an equality.

In the case where XX or YY has a point, it’s trivial. For suppose that XX has a point. Then we have a map Y1×YX×YY \cong 1 \times Y \to X \times Y, as well as the projection X×YYX \times Y \to Y. This gives YX×YYY \leq X \times Y \leq Y, and so

χ(X×Y)=χ(Y)=min{,χ(Y)}=min{χ(X),χ(Y)}. \chi(X \times Y) = \chi(Y) = \min \{\infty, \chi(Y)\} = \min \{\chi(X), \chi(Y)\}.

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 nn and towards the family of complete graphs. So here’s a first, mild, rephrasing.

Conjecture (Hedetniemi, version 2)   Let XX and YY be graphs and KK a complete graph. If there is a homomorphism X×YKX \times Y \to K, then there is a homomorphism XKX \to K or a homomorphism YKY \to K.

Writing KK as K nK_n, this just says: if χ(X×Y)n\chi(X \times Y) \leq n then χ(X)n\chi(X) \leq n or χ(Y)n\chi(Y) \leq n. That is, if χ(X×Y)n\chi(X \times Y) \leq n then min{χ(X),χ(Y)}n\min\{\chi(X), \chi(Y)\} \leq n. That is, min{χ(X),χ(Y)}χ(X×Y)\min\{\chi(X), \chi(Y)\} \leq \chi(X \times Y). 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 XX and complete graph KK, either K XK^X or K K XK^{K^X} 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 XX and YY, and a complete graph KK. We’ll prove the contrapositive of version 2: if there’s no homomorphism XKX \to K and no homomorphism YKY \to K then there is no homomorphism X×YKX \times Y \to K. Supposing that there are no homomorphisms XKX \to K or YKY \to K, version 3 implies that there are homomorphisms α:K XK\alpha: K^X \to K and β:K YK\beta: K^Y \to K. So we get homomorphisms

K X×Y(K X) Yα YK YβK K^{X \times Y} \cong (K^X)^Y \stackrel{\alpha^Y}{\longrightarrow} K^Y \stackrel{\beta}{\longrightarrow} K

The composite is a homomorphism K X×YKK^{X \times Y} \to K. So by the Exclusivity Principle applied to K X×YK^{X \times Y}, there’s no homomorphism X×YKX \times Y \to K.

Conversely, suppose that version 2 is true. Take a graph XX and a complete graph KK. The cartesian closed structure gives us a counit homomorphism

K X×XK, K^X \times X \to K,

(which concretely is just (θ,x)θ(x)(\theta, x) \mapsto \theta(x)). Version 2 then tells us that either there’s a homomorphism K XKK^X \to K or there’s a homomorphism XKX \to K. 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 XX and complete graph KK, at least one of the graphs X,K X,K K X,K K K X, X, K^X, K^{K^X}, K^{K^{K^X}}, \ldots has a point.

But that seems like just a dressing-up of version 3. Why? Well, for any graph AA, the cartesian closed structure gives us a unit map η A:AK K A\eta_A: A \to K^{K^A}. Taking A=XA = X and A=K XA = K^X in turn gives us maps

K (η X):K K K XK X,η K X:K XK K K X. K^{(\eta_X)}: K^{K^{K^X}} \to K^X, \qquad \eta_{K^X}: K^X \to K^{K^{K^X}}.

(Category theorists will now mumble the words “triangle identity”.) So K XK^X has a point iff K K K XK^{K^{K^X}} does. More generally, by the same argument, the nnth member of this sequence has a point iff the (n+2)(n + 2)th member does, for n2n \geq 2. So we might as well truncate the sequence to

X,K X,K K X. X, K^X, K^{K^X}.

We can also drop the XX, since the unit map η X:XK K X\eta_X: X \to K^{K^X} tells us that if XX has a point then K K XK^{K^X} does too. So we’re back to version 3.

Posted at December 5, 2014 1:18 PM UTC

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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.

Posted by: John Baez on December 5, 2014 4:31 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

I wish I had more success at communicating with graph theorists. I’ve been trying to do so ever since I started working on the magnitude of graphs, but with almost total failure. Just this week, I gave a colloquium at Birmingham (UK!), where there’s a big graph theory/combinatorics group. Though lots of them came to the talk and I explicitly appealed for their help during it, apparently I didn’t succeed in reeling them in. (At least, I heard nothing from them.)

Several other efforts of mine to involve graph theorists have failed. I don’t know whether my viewpoint is so different from theirs that I’m just not interesting them, or it’s just bad luck, or what.

Posted by: Tom Leinster on December 5, 2014 4:45 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Actually I regard just the sheer formulation of the conjecture in categorical terms, and moreover in a way that has some resonance in categorical logic, already a small victory.

What I mean is: it seems to me that a lot of things that excite graph theorists are somewhat opaque from the standpoint of expressing them in categorical language. For example, there is a much celebrated result, perhaps the most celebrated result in all of graph theory, called the Robertson-Seymour theorem. A graph HH is a minor of a graph GG if HH can be obtained from GG in finitely many steps where each step consists either in removing an edge, removing an isolated point, or contracting an edge (i.e. remove the edge and merge its endpoints). The minor relation is a partial order, and the Robertson-Seymour theorem says that the graph minor relation on the class of finite (simple loop-free) graphs has no infinite antichain. This result has useful consequences; for instance, if a class of graphs (for example, planar graphs) is closed under taking graph minors, then there exists a finite collection 𝒞\mathcal{C} of “forbidden minors”: the given class consists of exactly those graphs which have no element of 𝒞\mathcal{C} as a minor up to isomorphism – in the case of planar trees, the forbidden minors would be K 5K_5 and K 3,3K_{3, 3}.

But what is a graph minor, categorically? It doesn’t seem very obvious to me. It looks like a subquotient of some kind: certainly if HH is a subgraph of GG, then HH can be obtained from GG by removing edges and isolated points, whereas edge contraction looks more like a quotient construction.

To make this work, it seems convenient to formally identify a simple loop-free graph with a set carrying a reflexive symmetric relation. (Yes, it seems strange at first that loop-free graphs are identified with reflexive relations, but by analogy with categories, we can think of the “identity loop” at a vertex (enforced by reflexivity) as a path of length 00 which we don’t bother to draw. In any case, there is a canonical bijection between reflexive relations and irreflexive relations, and we can choose whichever is more convenient.) It’s convenient here to use reflexive relations because if we think of contracting an edge between two distinct points as a quotient, then that edge has to go somewhere under the quotient map, so take it to the identity loop at the merged point.

But of course edge contraction is but one sort of quotient; another would be simply to merge two points which didn’t have an edge between them. Is there some categorical way of picking out (series of) edge contractions? I don’t know. So here even just formulating graph minors in categorical terms could be tricky.

Posted by: Todd Trimble on December 24, 2014 12:26 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Here is, I think, the correct categorical definition of a contraction. Note first that for the category GraphGraph of undirected graphs (possibly with loops) we have a functor CC:GraphGraphCC: Graph \to Graph which takes each graph to its “set” of connected components represented as a discrete graph with loops at all vertices. The loops are essential because then we have a natural transformation ϕ:1 GraphCC\phi : 1_{Graph} \to CC which sends each vertex to its connected component.

GG' is a contraction of GG iff there is a map m:GGm: G \to G' which satisfies two conditions. 1) m is epi (which is the same as surjective in this category) and 2) m(x)=m(x)m(x) = m(x') implies that there is a path from xx to xx' in GG.

Now, consider any homomorphism f:KGf: K \to G. It’s not hard to see that im(f)im(f)’s connected components uniquely determine a contraction. I claim that GG' is a contraction of GG iff there is a pushout square composed of some KK, ff, m:GGm: G \to G', ϕ K:KCC(K)\phi_K: K \to CC(K), and q:CC(K)Gq: CC(K) \to G'. (Sorry, don’t know how to TeX commutative diagrams.) There is a lot to verify here but it all checks out. One has to use the fact that GCC(G)G \to CC(G') factors through CC(G)CC(G).

Now the definition applies to Top, FinTop, et al. I’m guessing Robertson-Seymour doesn’t hold in Graph? Does anyone have a counterexample? (Just to be safe, it holds for graphs possibly with loops, right? If not, replace GraphGraph above with the category of undirected graphs with loops.)

Posted by: Ibrahim Tencer on January 17, 2015 12:44 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Ibrahim, I’m glad you’re looking at this. I didn’t follow you when you said that epi means surjective in this category: it seems to me that the map from {0,1}\{0, 1\} (the discrete graph, say with loops) to {01}\{0 \leftrightarrow 1\} (add an edge to the preceding graph; it’s supposed to be undirected), the one which is the identity on vertices, is epi in the categorical sense, but not surjective on edges. It also satisfies condition 2).

If we strengthen “epi” to “regular epi” in condition 1), that would remove this counterexample. But I’m still not yet happy. Suppose we take a graph like this (and again let’s assume loops at every vertex):

01230 \leftrightarrow 1 \leftrightarrow 2 \leftrightarrow 3

and then we identify 00 with 33 to make a triangle. This is a regular epi and certainly satisfies condition 2). But it is not an edge contraction in any sense relevant to graph minors. So I’m not following you.

Finally, I’m not following your remark about Robertson-Seymour not holding in GraphGraph, because RS is a theorem about graphs! I think their graphs are simple graphs without loops, i.e. sets equipped with an irreflexive symmetric relation, but as I noted earlier, there is a bijection between irreflexive and reflexive relations, and I think we can add a loop to every vertex (to make the category theory work better) as long as we’re careful with language in the graph-theoretic statement.

Posted by: Todd Trimble on January 17, 2015 12:39 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Ah yes, surjective does not mean epi. But the definition with the pushout square should still correspond to the contractions: vertices are identified exactly when they are in the same connected component of the image of ff. So the example with identifying 0 and 3 to make a triangle should not have a pushout square like this: it would require that all vertices are identified because there has to be a path from 0 to 3 in the image of ff.

The process of going to irreflexive graphs to reflexive graphs by adding a loop at every vertex manifestly maintains the orders on subgraphs and contractions, so it shouldn’t change the existence of infinite antichains. But I guess that while contractions work more cleanly for reflexive graphs the definition doesn’t seem to require reflexivity on the face of it (if two different vertices are being identified, their image will have a loop, but if a vertex is left alone it doesn’t need to get a loop per se). And I guess the definition doesn’t even require the graph to be undirected either. So I was wondering if there is an obvious counter example to the theorem in these cases, or if it is feasible that it might still be true.

Posted by: Ibrahim Tencer on January 23, 2015 12:22 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Fascinating stuff! I’m particularly fond of your version 2 of the conjecture, and I’d like to reformulate it a little bit more.

To large extent, it seems that we don’t need to consider the category of graphs as a category. For graphs XX and YY, we’re only interested in whether a morphism XYX\to Y exists, and not so much in how many there are and how they compose with other morphisms. So, we might squash down the category of graphs to a proset of graphs, in which we have XYX\geq Y if and only if there is a morphism XYX\to Y; I like to think of a relation XYX\geq Y as the statement “XX can be turned into YY”.

Then, the categorical product becomes the join \vee, the coproduct the meet \wedge, and so we are actually dealing with a lattice. Moreover, I guess that the exponentials turn this lattice into a Heyting algebra?

With this, your version 2 of the conjecture can be phrased as asking whether XYKX\vee Y\geq K implies that XKX\geq K or YKY\geq K. In lattice-theoretic terminology, you are asking whether complete graphs are join primes in the lattice of finite graphs.

Posted by: Tobias Fritz on December 5, 2014 5:29 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Thanks!

I know what you mean by the order being what matters. As Gejza Jenča described rather winningly here, the underlying ordered set of the category of graphs is much more interesting than that of most other familiar categories. For example, it’s dense.

On the other hand, I’m not entirely ready to forget how many homomorphisms there are and pass to the underlying order. I can’t put my finger on why.

Posted by: Tom Leinster on December 5, 2014 5:39 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

By the way: (i) your order relation is the other way round from the one in my post, and (ii) I hope “proset” is a typo rather than an abbreviation for “preordered set”! It would be really confusing, as “pro” is already systematically used a prefix with a completely different meaning (pro-pp-group, etc).

Posted by: Tom Leinster on December 5, 2014 5:41 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Tom wrote:

your order relation is the other way round from the one in my post

Right, sorry about that. Feel free to edit my previous comment accordingly.

(The reason for me writing “XYX\geq Y” as representing XYX\to Y is that I like to think of XYX\geq Y as stating “if I have XX, then I can turn it into YY, and therefore XX is at least as good as YY”.)

I hope “proset” is a typo rather than an abbreviation for “preordered set”!

No, and in fact I thought that this was standard terminology — at least I’ve seen this term being used in this sense on many nLab pages. I hadn’t noticed the clash with other things “pro-” before, so thanks for pointing that out. Would you agree that the term “ordered set” is better, in the sense of a preset equipped with an ordering relation, plus the equality relation defined by the ordering?

Posted by: Tobias Fritz on December 5, 2014 5:56 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

I wrote:

Would you agree that the term “ordered set” is better

Just to clarify the reason for why I didn’t simply suggest “poset” in that context: I’m currently writing a paper in which the protagonists are ordered commutative monoids, in the sense of symmetric monoidal categories enriched over the Booleans. In order to make the terminology more uniform, I’d like to say things like this:

Definition: An ordered commutative monoid is an ordered set equipped with…

But I don’t want to turn this into a discussion on terminology, so I’ll stop here.

Posted by: Tobias Fritz on December 5, 2014 6:06 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

For some reason, the convention among category theorists is that when you turn a poset into a category, you put arrows aba \to b in the direction of inequalities aba \leq b. So when you turn categories into posets, it would make sense to do things the same way round. But of course, it’s an arbitrary convention. It sits well with situations where maps are injections (such as inclusions), but badly with situations where maps are surjections (such as the product-projections X×YYX \times Y \to Y that I used in my post). It’s not important!

I hadn’t heard “proset” before. Maybe I over-reacted to the potential for confusion with “pro” as in “pro-object”; after all, people say “profunctor” without anyone getting confused. As for “preset”, I think I’ve only heard this from nLab users. I don’t understand the definition given on the nLab page. Previously I’d understood it to mean “set equipped with an equivalence relation”, but there’s probably some subtle point of logic that I’m not getting.

Posted by: Tom Leinster on December 6, 2014 11:55 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

I don’t understand presets, but they’re cool. A preset is a gizmo that consists of a rule for ‘generating members’ of it, but not necessarily a rule for proving two members are equal. If you have the latter, and it’s well-behaved, you have a set.

It’s somewhat going against the point of this idea to formalize presets using set theory; the goal seems to be to formalize sets as special presets. This is why it’s a bit hard for set-based mathematicians to understand presets.

Posted by: John Baez on December 15, 2014 2:40 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

One reason why I like this reformulation of Hedetniemi’s conjecture —

for every graph XX and complete graph KK, either K XK^X or K K XK^{K^X} has a point

— is that it involves something like double duals, which I’ve been thinking about a lot lately.

There’s a cute way to put this conjecture into words.

Remember that the points of the graph K n XK_n^X are the nn-colourings of XX. This graph K n XK_n^X might not have any points, but it still makes sense to refer to it as “the graph of nn-colourings of XX”, just as it makes sense to talk about pointless locales such as “the locale of surjections \mathbb{N} \to \mathbb{R}” and “the locale of random sequences of 0s and 1s”.

So, the conjecture says:

for every graph XX and n0n \geq 0, either XX has an nn-colouring, or the graph of nn-colourings of XX has an nn-colouring.

Posted by: Tom Leinster on December 7, 2014 12:03 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Just to get a bit of exercise, let’s check that the conjecture holds for complete graphs XX. This ought to be easy…

We have to show that for complete graphs KK, either K XK^X or K K XK^{K^X} has a point.

If XX has no more vertices than KK, it’s a pushover. You simply choose an injection V(X)V(K)V(X) \to V(K), and that’s a graph homomorphism. So then, K XK^X has a point.

If XX has more vertices than KK then clearly X KX^K has no points, so we’re going to have to show that K K XK^{K^X} has a point. Constructing elements of a double dual is often interesting. Think about vector spaces or ultrafilters, for instance.

It turns out that when XX has more vertices of KK, there is a point of K K XK^{K^X} that can be described as “choose a nontrivial fibre”. More exactly, we can define a homomorphism Φ:K XK\Phi: K^X \to K as follows: for any vertex f:V(X)V(K)f: V(X) \to V(K) of K XK^X, choose an element Φ(f)V(K)\Phi(f) \in V(K) such that |f 1(Φ(f))|>1. \left|f^{-1}\bigl(\Phi(f)\bigr)\right| \gt 1. There’s always at least one such element, as XX has more vertices than KK. A little check reveals that Φ\Phi is a homomorphism — the kind of check that’s actually quite pleasant to do, but is never pleasant to read.

Posted by: Tom Leinster on December 7, 2014 12:17 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Or to put it more dramatically, Hedetniemi’s conjecture is:

Every graph either has an nn-colouring, or its graph of nn-colourings has an nn-colouring.

That doesn’t sound like it makes sense: if XX has no nn-colouring, surely its graph of nn-colourings is empty? Nope — it’s just pointless.

Posted by: Tom Leinster on December 7, 2014 8:04 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Here’s a rather obvious question on trying to generalize the conjecture that you’ve certainly considered already. I haven’t thought about it much, so this might be totally naive. In any case, I’d be curious to know what your thoughts are.

You’ve defined the category of graphs as the category of internal symmetric relations on FinSet. So what if we make the analogous definition in some other regular or Barr-exact category? Can we find a sensible generalization of the conjecture?

The critical issue is to ask what the analogues of the complete graphs might be. In a topos, we could possibly define the complete graph over any object AA by taking the complement of the diagonal AA×AA\to A\times A to be its object of edges, where I mean “complement” in the sense of negating its characteristic function. Is this sensible? Should we also have to restrict the question to objects AA that are finite in some sense?

So, in order to get some idea about how to attack the conjecture, maybe one can try to generalize it first and get some feel for how far one can push generalizations before they are easily seen to be false.

On a different note, there is the De Bruijn–Erdős theorem which states that an infinite graph can be coloured with kk\in\mathbb{N} colours if and only if every finite subgraph can. The proof seems related to the fact that an infinite graph is the colimit of its finite induced subgraphs, but I haven’t stared at it long enough to say for sure. In any case, gluing graphs together by taking colimits is something that one might want to do in graph theory, and in order to do so one really needs to look at the category of graphs instead of just the poset of graphs.

Posted by: Tobias Fritz on December 7, 2014 9:27 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

This post:

says:

When can we properly color the vertices of a graph with a few colors? This is a notoriously difficult problems. Things get a little better if we consider simultaneously a graph together with all its induced subgraphs. Recall that an induced subgraph of a graph GG is a subgraph formed by a subset of the vertices of GG together with all edges of GG spanned on these vertices.

Maybe there’s a category-theoretic perspective on this? Are we replacing the graph by some sort of poset of subobjects? (Not all subobjects, apparently.)

Posted by: John Baez on December 21, 2014 3:17 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Thanks for the link. I guess the induced subgraphs are the regular monics: in other words, the maps that arise as equalizers.

Here’s a question for someone like you who knows lots about logic.

In linear logic, one often thinks of the unit object II of the monoidal category as “falsity”, and of Hom(X,I)\mathbf{Hom}(X, I) as “not II”. Here I’m imagining that we’re in a symmetric monoidal closed category, and writing Hom\mathbf{Hom} for the internal hom.

Hedetniemi’s conjecture states that for any graph XX and complete graph KK, either K XK^X or K K XK^{K^X} has a point. Now, KK isn’t the unit of any monoidal structure that I know of, and besides, there are lots of complete graphs KK. Nevertheless, maybe it’s possible to construe the statement

either K XK^X or K K XK^{K^X} has a point

as

either ¬X\neg X or ¬¬X\neg \neg X has a proof.

The fact that K XK^X and K K XK^{K^X} can’t both have a point encourages me in this, as does the fact that K XK^X has a point if and only if K K K XK^{K^{K^X}} has a point.

So the question is this. I know there are lots of logics in which P¬PP \vee \neg P fails, for some statements PP. But in such logics, is it usually the case that ¬P¬¬P\neg P \vee \neg\neg P holds?

(These matters are delicate — probably the answer to the question I just asked depends on exactly how I phrased it. But I’m also interested in answers to differently-phrased questions in the same spirit. So don’t feel the need to interpret the question too literally.)

Posted by: Tom Leinster on December 21, 2014 5:59 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Just to clarify, the mention of linear logic was incidental. My question —

In logics where P¬PP \vee \neg P fails, is it usually the case that ¬P¬¬P\neg P \vee \neg\neg P holds?

— was about logics of all kinds. And my idea of interpreting Hedetniemi’s conjecture as a statement about multiple negations wasn’t related to linear logic either.

Posted by: Tom Leinster on December 21, 2014 10:19 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

So the question is this. I know there are lots of logics in which P¬PP \vee \neg P fails, for some statements PP. But in such logics, is it usually the case that ¬P¬¬P\neg P \vee \neg\neg P holds?

It’s actually fairly rare. Even though it’s not a very strong principle. At least, it’s weaker than the well-ordering of the propositions (AB)(BA)(A\rightarrow B)\vee (B\rightarrow A). So there are, I think, plenty of toposes where it holds but the excluded middle doesn’t.

What you always have is ¬¬(P¬P)\neg\neg(P\vee\neg P) (independently, even of the choice of negation, so ¬A\neg A can be chosen to mean AKA\rightarrow K for an arbitrary KK, not that it’s terribly useful for the case at hand).

It’s an entertaining thought, however. As it leads to rephrase the conjecture, in a more logical style, as K X+K K XK^X + K^{K^X} has a point for all KK.

Posted by: Arnaud Spiwack on December 22, 2014 12:51 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Thanks! I think I had that fact about ¬¬(P¬P)\neg\neg(P \vee \neg P) in the back of my mind, without remembering it accurately.

Posted by: Tom Leinster on December 22, 2014 1:00 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Maybe it is useful, then, to note that ¬¬(P¬P)\neg\neg(P\vee\neg P) is equivalent to ¬(¬P¬¬P)\neg(\neg P \wedge \neg\neg P), so it is an instance of ¬(Q¬Q)\neg(Q\wedge \neg Q) which, fortunately, holds.

Personally, though, I like to see it more operationally: a function of type (A+(AK))K(A+(A\rightarrow K))\rightarrow K (an arrow in some ccc with finite sums), is both a function AKA\rightarrow K and a function (AK)K(A\rightarrow K)\rightarrow K. Composition gives us a KK.

Of course both amount to the same thing. But one of these may be easier to remember.

Posted by: Arnaud Spiwack on December 23, 2014 10:27 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

I’m not sure what to make of it, but your version 2 of the Hedetniemi conjecture actually reminds me a bit of the “focalisation” property of linear logic, which is a way of saying that the inversion properties of the connectives can in a certain sense be turned around. For example, in the case of additive conjunction A&BA\& B, the inversion property says that the &\&-right rule sending [a pair of a proof of ΓA\Gamma \vdash A and a proof of ΓB\Gamma \vdash B] to [a proof of ΓA&B\Gamma \vdash A\& B] witnesses an isomorphism. Turning to the other side of the turnstile, in general of course it is not true that [a proof of A&BΔA \& B \vdash \Delta] is [either a proof of AΔA \vdash \Delta or a proof of BΔB \vdash \Delta]. However, the idea behind focusing is that we can “focus” on the formula on the left side of the turnstile A&B̲Δ\underline{A\& B} \vdash \Delta and temporarily enter a situation where this isomorphism holds: that is, by definition, a proof of A&B̲Δ\underline{A\& B} \vdash \Delta is either proof of A̲Δ\underline{A} \vdash \Delta or a proof of B̲Δ\underline{B} \vdash \Delta. (The original interest of this was that focusing proof search is complete: that is, any sequent which is provable can be built bottom-up by a repeated cycle of 1. apply invertible rules, 2. pick a formula somewhere in the sequent to focus on, 3. apply focusing rules.)

One easy consequence of focalization (it is also an easy consequence of cut-elimination) is that when the right-hand side Δ\Delta consists purely of atomic formulas, any proof of A&BΔA \& B \vdash \Delta is either a proof of AΔA \vdash \Delta or a proof of BΔB \vdash \Delta. Which sounds a bit like your version 2, though whether it is of any formal relevance I can’t say…

Posted by: Noam Zeilberger on December 23, 2014 8:38 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

I might add that the category of graphs (sets with symmetric relations) is not just a cartesian closed category: it’s a quasitopos. In more detail, one may calculate that it’s the category of separated presheaves for the ¬¬\neg\neg-topology on the topos of presheaves on the category 121 \stackrel{\to}{\to} 2, or the site of finite sets 11 and 22 and injections between them (I’m copying down what I wrote in the nLab article some while back).

One immediate reason for making this comment is that in a quasitopos, one has a classifier for regular monics (not for all monics), so that it would probably be fortuitous if the right notion of subobject for induced subgraph turns out to be regular subobject, as Tom is surmising.

There are some little points to make with regard to linear logic. The term “linear logic” is often used these days in a pretty broad sense, embracing more than the “classical” linear logic first outlined by Girard whose categorical semantics is naturally valued in a *\ast-autonomous category. The classical models were really “classical”, in that Girard wanted to retain the law of excluded middle for his so-called multiplicative linear logic, or MLL. Here MLL is interpreted using two monoidal structures, one called “conjunction” which has operations (,I)(\otimes, I) [where II = “true”], and one called “disjunction” which has operations (,D)(\wp, D) [where DD = “false”; I forget the standard way of writing that funny ‘par’ symbol these days, which stands for disjunction], taking DD to be the dualizing object for the *\ast-autonomous structure. If we interpret linear negation as the contravariant functor () *()D(-)^\ast \coloneqq (-) \multimap D, then one can define ABA \wp B to be (A *B *) *(A^\ast \otimes B^\ast)^\ast, and one can witness excluded middle via a canonical map IA *AI \to A^\ast \wp A, dual to a map AA *DA \otimes A^\ast \to D.

What Tom was referring to looks more to me like some form of what is called multiplicative intuitionistic linear logic, or MILL. Here we don’t have an involutory negation that interchanges two monoidal structures. Often the categorical semantics is taken to be a symmetric monoidal closed category with monoidal product interpreting conjunction and monoidal unit interpreting “true”, but there is some wiggle room in what we might consider as playing the role of “false”, for example maybe some object like a complete graph KK. I don’t have any good ideas about this at the moment, but I did want to clarify that “linear logic” can be considered as a rubric for a pretty broad umbrella, and one should be prepared to be flexible and not doctrinaire as to the meaning of the term.

Posted by: Todd Trimble on December 21, 2014 8:28 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Thanks for the generous and learned interpretation of what I wrote about linear logic! The memory of trying to write upside-down ampersands while taking notes in seminars I didn’t understand is pleasantly nostalgic for me.

I know you mentioned before that the category of (reflexive) graphs is a quasitopos. At the time, I didn’t know what to do with that thought. I’m not sure I do now, but this observation of yours did come back into my mind as I began to try out this logical interpretation of K XK^X.

Posted by: Tom Leinster on December 21, 2014 8:57 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Turn your notes upside down and they might make more sense.

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Posted by: John Baez on December 21, 2014 10:06 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

I think Todd said everything about linear logic better than I would have. The fact that there are lots of complete graphs KK, most of which don’t seem to be units for interesting tensor products, makes me shy away from thinking of them as meaning ‘false’ in some sort of linear logic, or K XK^X as a kind of ‘negation’ of the graph XX.

Nonetheless, I couldn’t help noting that both Tom and Todd mentioned double negation. Tom was talking about improving the logic of graph theory, making it more classical, by replacing each graph XX with some sort of ‘double negation’ K K XK^{K^X}. This is like Gödel’s old trick for embedding classical logic in intuitionistic logic through double negation: when the classical logician says “XX”, the intuitionistic logician should interpret it as meaning “not not XX.” In the case of graph theory, we don’t know that either XX or its ‘negation’ has a point, but we know that either the double negation K K XK^{K^X} or its negation has a point. So we’re getting a ‘law of excluded middle’ in an intuitionistic context just as Gödel did.

Later, Todd mentioned that the quasitopos of graphs could be defined using a ‘double negation’ topology. Are these ideas truly related? It would be really cool, but I don’t see how.

I’m a bit more optimistic about the idea that graphs (sets with symmetric relations) form a quasitopos, and the induced subgraphs i:XYi: X \hookrightarrow Y are exactly the strong monomorphisms.

(Is the second part true? I think so. Todd and Tom said ‘regular’ monomorphisms, but it’s the strong monomorphisms that are classified by the weak subobject classifier of a quasitopos.)

What did Gil Kalai mean, exactly, by saying it’s “better if we consider simultaneously a graph together with all its induced subgraphs”? We’d have to look at his results and psychoanalyze them to see if there’s a slick category-theoretic way to state this perspective. But that would take work, so maybe first we can ask: is there some standard way to take a topos (or quasitopos) and form a new category of posets, e.g. Heyting algebras (or “quasi-Heyting algebras”, whatever those might be), by taking each object and replacing it with its poset of (strong) subobjects? Obviously there are certain contexts where doing this doesn’t really change anything, like going from the category SetSet to the category of complete Boolean algebras. But maybe there are contexts, and ways to do this trick, that somehow “improve” the original category.

Posted by: John Baez on December 21, 2014 9:31 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Here’s a simple way to get some ideas. As you both probably know, Lawvere had a schtick about how the graphs category theorists like best — the directed multigraphs—form a presheaf category on the category 121 \stackrel{\to}{\to} 2. The fun part of this schtick is that the subobject classifier is a graph, where the two usual truth values — true and false — become vertices, but we also have some edges with nice meanings like “going from truth to truth while staying true”, “going from truth to truth while becoming false”, “going from truth to falsity”, and so on. (Lawvere had other names for these, like “sojourn”.)

One can try to learn to think using this kind of logic: that is, think using the internal logic of the topos of directed multigraphs.

Similarly, one can try to learn to think using the internal logic of the quasitopos of graphs that Tom is studying here: sets with symmetric relations. How does the logic change? There’s at most one way to go from truth to truth, or truth to falsity, etc. And whenever you can go from one to the other, you can go back.

Can we intuitively understand what Todd said, that this quasitopos is the category of separated presheaves on 121 \stackrel{\to}{\to} 2 with a ‘double negation topology’? Obviously this sets up some functors going between the topos of direct multigraphs and the quasitopos of sets with symmetric relations. But what are these functors like in terms of logic? How is this ‘double negation topology’ related to double negation in logic?

And what’s this trick of ‘making the logic more classical’ by replacing objects XX by objects K K XK^{K^X}, where KK is a complete graph on nn vertices?

What’s the ‘logical’ way of thinking about KK?

Is a strong subobject i:KXi : K \to X what graph theorists call an nn-element clique? If so, I guess we’re saying ii is a kind of proposition about vertices of XX, namely that they’re in some nn-element induced graph, with the property that you can go between any two vertices where this is proposition is true, using an edge of the sort “going from true to true”.

It would take a while to learn to think in the internal logic of this quasitopos, and only certain people could do it, but a fair fraction of them are reading this blog.

Posted by: John Baez on December 21, 2014 10:02 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Lots here to think about, obviously, but let me just say one thing in response to this:

What’s the ‘logical’ way of thinking about KK?

As I said in the post, complete graphs are in some sense maximal pointless graphs. “Pointless” means that there’s no map 1K1 \to K; or in the language I’m now trying out, that there’s no proof of KK. “Maximal” means that there are as many edges as possible, given that it’s pointless and has the prescribed number of vertices. (It would be nice to put that into logical language.)

The fact that “there’s no proof of KK” is another justification for thinking of KK as “false”.

It’s clear that I don’t know how to make any of this precise!

Posted by: Tom Leinster on December 21, 2014 10:15 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Tom wrote:

The fact that “there’s no proof of KK” is another justification for thinking of KK as “false”.

You may have talked about this before, but there’s a functor

hom(1,):GraphSet hom(1,-) : Graph \to Set

where 11 is the terminal object, and this sends “pointless” objects to the empty set. That would be one obvious way to formalize your idea. What’s this functor like and how does it interact with

K :Graph opGraph? K^{-} : Graph^{op} \to Graph \quad ?

Posted by: John Baez on December 22, 2014 2:27 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

What’s the functor hom(1,):GraphSethom(1, -): Graph \to Set like? Well, it sends a graph to its set of points (loops). It preserves limits, like all representables. It’s neither full nor faithful.

How does it interact with K :Graph opGraphK^{-}: Graph^{op} \to Graph? Well, the composite functor Graph opSetGraph^{op} \to \Set sends a graph XX to the set hom(X,K)hom(X, K) of KK-colourings of XX.

Am I answering the kinds of question you had in mind?

Posted by: Tom Leinster on December 22, 2014 12:47 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Just a small correction:

In the case of graph theory, we don’t know that either XX or its ‘negation’ has a point, but we know that either the double negation K K XK^{K^X} or its negation has a point.

We don’t know the latter: that’s exactly Hedetniemi’s conjecture!

The statement “either XX or K XK^X has a point” is demonstrably false for some values of XX and KK. E.g. if XX is a graph with no loops and KK is the complete graph on some number of vertices smaller than the chromatic number of XX, then the statement fails.

The statement “either K K XK^{K^X} or K K K XK^{K^{K^X}} has a point” is equivalent to “either K XK^X or K K XK^{K^X} has a point” (since K XK^X has a point iff K K K XK^{K^{K^X}} does). And that last statement in quotation marks is one of the versions of Hedetniemi’s conjecture stated in my post.

Posted by: Tom Leinster on December 21, 2014 10:25 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Tom wrote:

We don’t know the latter: that’s exactly Hedetniemi’s conjecture!

Right. I meant that in this dream you’re cooking up, that’s how things go. But the word ‘know’ was the wrong one.

Posted by: John Baez on December 22, 2014 2:13 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

You’re trying to prove that hom(1,)hom(1,-) and K K^- have a rather remarkable interaction, namely either hom(1,K X)hom(1,K^X) or hom(1,K K X)hom(1,K^{K^X}) is nonempty. So I thought that listing all the ways in which these functors get along—either for any object KK in any quasitopos, or especially in this one, or especially for these particular objects KK—would be helpful. The ones you’ve listed so far aren’t very promising yet; mostly they’re the reasons I asked the question. I was hoping there would be some more tricky and surprising facts already known.

Posted by: John Baez on December 23, 2014 3:53 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Ah, I see what you mean now. Thanks.

(Just logged on to see that lots of people have left interesting comments…)

Posted by: Tom Leinster on December 24, 2014 4:04 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

One category-theoretic line of indirect attack would be this: define an object KK in a quasitopos (e.g. a topos) to be false if either hom(1,K X)hom(1,K^X) or hom(1,K K X)hom(1,{K^K}^X) is nonempty for every object XX. What are the most interesting examples of false objects? What do they have in common?

Posted by: John Baez on December 24, 2014 3:41 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Did you mean to say exactly what you said? Wouldn’t 11 be ‘false’ in that case?

Posted by: Todd Trimble on December 24, 2014 3:58 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Actually, come to think of it, every object would be false in that case, since we can take XX to be KK and get one of those homs to be nonempty. But maybe you slipped in a typo.

Posted by: Todd Trimble on December 24, 2014 4:00 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Not quite, because John wanted hom(1,K X)\mathrm{hom}(1,K^X) or hom(1,K K X)\mathrm{hom}(1,K^{K^X}) to be nonempty for every XX. But I agree that 11 itself is trivially “false” in John’s sense. More generally, every KK which has a point is “false”, since then we have a composite X1KX\to 1\to K.

Posted by: Tobias Fritz on December 24, 2014 4:11 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Oops, yes; sorry.

Posted by: Todd Trimble on December 24, 2014 5:13 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Moreover, if XX has a point, then evaluation at that point yields a morphism K XKK^X\to K. Therefore, when checking John’s condition, it is enough to consider ‘pointless’ XX.

Posted by: Tobias Fritz on December 24, 2014 4:27 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Okay, I was shooting from the hip and shot myself in the foot, but maybe it’s only a flesh wound. To capture the concept I was vaguely imagining, perhaps I should redefine a false object to be an object KK with no points, such that for every object XX either K XK^X or K K XK^{K^X} has a point.

It’s embarrassing, but I need to check: in a topos, or more generally a quasitopos, is 00 always false in this sense?

I’m trying to capture Tom’s idea that Hedetniemi’s conjecture says any complete graph behaves in some important ways like the empty graph, also known as 0. It seems they both act like ‘false’ in intuitionistic logic.

(It’s obvious but someone must come out and say it: the empty graph is an example of a complete graph.)

Posted by: John Baez on December 25, 2014 5:40 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Sorry to shoot this down, but unfortunately I do not think 00 is a false object in this new sense.

Let’s remind ourselves that in a quasitopos or even in any cartesian closed category that has an initial object, 00 is a what is called a strict initial object, which means: if an object XX maps to 00, X0X \to 0, then XX is isomorphic to 00. (Proof: given a map f:X0f: X \to 0, we can manufacture a map 1 X,f:XX×0\langle 1_X, f\rangle: X \to X \times 0. In the other direction, we have a projection map X×0XX \times 0 \to X left inverse to the previous map, i.e., the composite of these two maps is 1 X1_X. But also, in a cartesian closed category, X×X \times - is left adjoint to an exponential functor () X(-)^X, and like any left adjoint it preserves all colimits, including the empty colimit 00. Thus X×00X \times 0 \cong 0. Thus XX×00X \to X \times 0 \cong 0 has a left inverse !:0X!: 0 \to X, and !:0X!: 0 \to X has a left inverse f:X0f: X \to 0 (by initiality of 00), and so !! and ff are inverse to one another.)

Armed with this fact, let K=0K = 0 and let XX be any object. If K XK^X has a point, naming a map XK=0X \to K = 0, then X0X \cong 0. In fact K XK^X has a point iff X0X \cong 0. But now let’s consider the topos Set/2Set/2 where 2=1+12 = 1 + 1 – this is the same as the topos Set 2Set×SetSet^2 \simeq Set \times Set if you prefer – and let’s take XX to be the object 121 \to 2 given by the first coproduct inclusion 11+11 \to 1 + 1. (Or the object (1,0)(1, 0) in Set×SetSet \times Set if you prefer.) By the above, K XK^X does not have a point. Now let YY be the object 121 \to 2 given by the second inclusion, or (0,1)(0, 1) if you prefer. Then X×Y0=KX \times Y \cong 0 = K in this topos, which curries (or should it be curryes?) to a map YK XY \to K^X. If K K XK^{K^X} has a point, naming a map K XK=0K^X \to K = 0, then we’d be forced to have K X=0K^X = 0 by the previous paragraph, and since we also have a map YK X=0Y \to K^X = 0, we’d be forced into Y=0Y = 0, which is a contradiction.

That was probably phrased more circuitously than it might have been, but I hope it’s followable.

I haven’t thought enough about all this to propose a possible fix yet.

Posted by: Todd Trimble on December 25, 2014 6:33 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Todd wrote:

I haven’t thought enough about all this to propose a possible fix yet.

Could you explain a bit more what the problem is with 00 not being false? Is it just that the term “false” might be a bit off the mark, or is there more to it? Your argument shows that 00 is not necessarily a false object for both the original definition and the revised one, doesn’t it?

I have two more simple observations, still in terms of the original definition of false objects.

First observation: if KK is false, and KK' is such that there are morphisms KKK\to K' and KKK'\to K, then KK' is false as well.

Proof: By currying, KK is false whenever for every XX there exists XKX\to K or K XKK^X\to K. If the former happens for a given XX, then we also have the composite XKKX\to K\to K'; if it is the latter K XKK^X\to K, then we have the composite K XK XKK, K'^X \to K^X \to K \to K', where the first arrow comes from functoriality of exponentials. Hence KK' is false as well.

Second observation: In order to talk about false objects, the structure of a category is more than we actually need! In fact, it is enough to consider the preordered set P(C)P(C) determined by the category CC. If CC is a quasitopos, then P(C)P(C) is a Heyting algebra, since the bijection C(X×Y,Z)C(X,Z Y)C(X\times Y,Z)\cong C(X,Z^Y) shows that the functor CP(C)C\to P(C) preserves exponentials. Therefore, KK is false in CC if and only if it is false in P(C)P(C).

Note that this makes the first observation even more trivial than it already is, because having arrows KKK\to K' and KKK'\to K makes the two objects isomorphic in P(C)P(C).

In a Heyting algebra, we can reformulate the definition of false objects a bit further, using the observation that an element (object?) in a Heyting algebra has a point if and only if it is a top element. This implies that KK is false if and only if K XK K X1 K^X \vee K^{K^X} \cong 1 for every XX. To me, this suggests that we’re not doing much category theory here, but mostly order theory. In fact, this point of view simplifies Todd’s counterexample significantly: PP(Set/2) is the poset whose Hasse diagram looks like a diamond, with bottom and top elements 00 and 11, and intermediate elements XX and YY that are not ordered relative to each other. Todd’s argument shows that this poset is itself a quasitopos in which 00 is not a false object!

Posted by: Tobias Fritz on December 25, 2014 10:39 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Tobias asked:

Could you explain a bit more what the problem is with 00 not being false?

Maybe no problem! But John seemed to want this, and in any case he asked the question, so I decided to answer.

Posted by: Todd Trimble on December 25, 2014 12:55 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Todd wrote:

Sorry to shoot this down, but unfortunately I do not think 00 is a false object in this new sense.

Keep on shooting, I’ll just deal with it.

Tobias asked:

Could you explain a bit more what the problem is with 00 not being false?

In very ‘classical’ categories like SetSet the principle of excluded middle kicks in and for any object, either XX or 0 X0^X has a point (and only one does). This fails in more ‘intuitionistic’ categories. But sometimes — and I see I’m quite unclear about exactly when — we can rescue the situation: either 0 X0^X or 0 0 X0^{0^X} has a point. I over-optimistically hoped it would be true in every quasitopos, so that then we could march on and ponder other objects KK in quasitopoi where either K XK^X or K K XK^{K^X} has a point… as a warmup to studying Hedetniemi’s conjecture.

Later I saw Arnaud Spiwack’s remark so I realized my hope had to be trimmed down:

Tom wrote:

So the question is this. I know there are lots of logics in which P¬PP \vee \neg P fails, for some statements PP. But in such logics, is it usually the case that ¬P¬¬P\neg P \vee \neg\neg P holds?

Arnaud replied:

It’s actually fairly rare. [… But] there are, I think, plenty of toposes where it holds but the excluded middle doesn’t.

So now I’m curious about when it holds.

Posted by: John Baez on December 25, 2014 8:42 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

John said:

So now I’m curious about when it holds [that for every XX, either 0 X0^X or 0 0 X0^{0^X} has a point].

By putting together Tom’s reasoning with Todd’s, we can characterize all those cartesian closed categories with initial object in which this holds: they are precisely those CCCs that do not have ‘zero divisors’, in the sense that X×Y0X\times Y\cong 0 implies X0X\cong 0 or Y0Y\cong 0.

Proof: Going back to Tom’s original post and the proof of the equivalence between versions 2 and 3 of Hedetniemi’s conjecture, you’ll see that the statement “for every XX, either 0 X0^X or 0 0 X0^{0^X}” is equivalent to “if there is a morphism X×Y0X\times Y\to 0, then there is a morphism X0X\to 0 or Y0Y\to 0”. But by Todd’s comment on strictness of initial objects, this is equivalent to “if X×Y0X\times Y\cong 0, then X0X\cong 0 or Y0Y\cong 0”. QED.

As a nice family of examples, let’s consider the topos of sheaves on a topological space.

Claim: The topos of sheaves on a space does not have zero divisors if and only if the space is hyperconnected (also called irreducible).

Proof (of the contrapositive): Suppose we have XX and YY with X×Y0X\times Y\cong 0, but X0X\ncong 0 and Y0Y\ncong 0. This means that there are nonempty opens UU and VV with X(U)X(U)\neq \emptyset and Y(V)Y(V)\neq \emptyset. But then both XX and YY also have a section over UVU\cap V, and then so does X×YX\times Y. By X×Y0X\times Y\cong 0, this forces UV=U\cap V=\emptyset. (Recall that every sheaf has exactly one section over the empty set, including the empty sheaf.) Hence UU and VV are disjoint and nonempty opens, so that the space is not hyperconnected.

Conversely, if the space is not hyperconnected, then we have disjoint nonempty opens UU and VV, and their indicator sheaves have the empty sheaf as their product. QED.

So, the category of sheaves on a space has the propery that ‘00 is false’ in John’s sense if and only if the space is hyperconnected. Not very surprisingly, the same statement and proof also apply to the topos of presheaves. (And also to the quasitopos of separated presheaves, if products of these can be computed as (X×Y)(U)=X(U)×Y(U)(X\times Y)(U)=X(U)\times Y(U), but I’m not sure about this.)

Posted by: Tobias Fritz on December 26, 2014 9:20 AM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

With regard to Tobias’s last paragraph: separated presheaves form a reflective subcategory of the presheaf category, so the inclusion SepPreShPreShSepPreSh \hookrightarrow PreSh preserves all limits, and so products in particular can be computed as he says. See the nLab article.

It’s also true that the reflector preserves some finite limits such as products. A paper by Garner and Lack gives more details on this.

Posted by: Todd Trimble on December 26, 2014 1:16 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Tobias wrote:

We can characterize all those cartesian closed categories with initial object in which [for every XX, either 0 X0^X or 0 0 X0^{0^X} has a point]: they are precisely those CCCs that do not have ‘zero divisors’, in the sense that X×Y0X\times Y\cong 0 implies X0X\cong 0 or Y0Y\cong 0.

That’s very nice! One can imagine the latter as a plausible logical principle, akin to “if X&YX \& Y is false, either XX is false or YY is false” — a principle which of course can fail if both XX and YY hold ‘somewhere’, but only in non-overlapping locations, as we’d get in a topos of presheaves on a space with two disjoint nonempty open sets.

That makes your followup very nice too: the topos of presheaves (or separated presheaves, or sheaves) on a space has zero divisors iff the space fails to be hyperconnected — that is, iff it contains two disjoint nonempty open sets.

So now I can’t resist asking about the generalization from spaces to sites? Have people defined ‘hyperconnectedness’ for sites?

If so, I guess it must be true that graphs of Tom’s sort are separated presheaves on a ‘hyperconnected’ site. (Todd has already described the site.)

If so, we can think of Hedetniemi’s conjecture as some sort of strengthening of hyperconnectedness, where some nice property of OO is shared by all complete graphs.

Have people looked at such strengthenings? Can we?

Posted by: John Baez on December 27, 2014 6:38 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

John asked:

(Is the second part true? I think so. Todd and Tom said ‘regular’ monomorphisms, but it’s the strong monomorphisms that are classified by the weak subobject classifier of a quasitopos.)

Happily, in a quasitopos the regular monomorphisms and strong monomorphisms coincide. So we’re okay there. (I don’t know who wrote that into the nLab, but anyway I think the result could probably be found in Wyler’s notes.)

By the way, my picture of the site was slightly wrong. The words were right, but my picture should have included a loop 2\stackrel{2}{\circlearrowright} to indicate the nontrivial automorphism on 22 that interchanges co-source and co-target – that’s the baby that puts the “symm” in “symmetric graph”. Without that automorphism, the separated presheaves on the presheaf topos would be the category of simple directed graphs (i.e., sets equipped with a binary relation).

Posted by: Todd Trimble on December 21, 2014 11:08 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Happily, in a quasitopos the regular monomorphisms and strong monomorphisms coincide. So we're okay there. (I don't know who wrote that into the nLab, but anyway I think the result could probably be found in Wyler's notes.)

I think I remember that Wyler glosses over this very quickly. But it’s just a matter of writing the pullback diagram for the classifier as an equivalent equaliser diagram (a classified mono f:ABf:A\rightarrow B equalises its characteristic function χ f\chi_f and the top element :BA\top:B\rightarrow A).

In fact Wyler’s definition of a quasitopos is a topos where some collection of monos is classified (it follows that the classified monos are precisely the equalisers, which includes (hence equals) the strong monos).

Posted by: Arnaud Spiwack on December 22, 2014 12:34 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

OK, so we’ve got the subcategory CC of SetSet consisting of the objects 11 and 22 and the injections between them.

A presheaf on CC consists of a directed multigraph EVE \stackrel{\to}{\to} V together with an involution σ\sigma of EE that exchanges source and target. That almost amounts to an undirected multigraph (with each undirected edge represented by a pair of arrows in EE in opposite directions). But not quite, as we have to think about the behaviour of the involution σ\sigma on loops. It might fix some of them.

I haven’t tried to work through what the separated presheaves for the double negation topology are, but you’re telling me that they’re exactly the symmetric graphs, i.e. sets equipped with a symmetric binary relation. I guess that means they’re just those presheaves such that for all vertices xx and yy, there’s at most one directed edge from xx to yy. Is that right?

Posted by: Tom Leinster on December 22, 2014 12:58 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

Is that right?

That’s right. Actually the calculation is easy as pie. But since I seem to rehearse this type of argument to myself every few months or so, I might as well write it down somewhere I can find it again if I’m in a hurry.

So let CC be this category of finite sets 11, 22 and injections between them. You can think of a presheaf on CC as a directed graph equipped with a direction reversing involution on the set of edges. We want to determine those presheaves X:C opSetX: C^{op} \to Set such that, for every ¬¬\neg\neg-dense sieve i:Fhom(,c)i: F \hookrightarrow \hom(-, c), the induced map

X(c)Set C op(hom(,c),X)i *Set C op(F,X)X(c) \cong Set^{C^{op}}(\hom(-, c), X) \stackrel{i^\ast}{\to} Set^{C^{op}}(F, X)

is monic. Being ¬¬\neg\neg-dense means that ¬¬F\neg\neg F, as a subobject of hom(,c)\hom(-, c), is all of hom(,c)\hom(-, c). Which is the same as saying ¬F=0\neg F = 0 – or that the only subgraph that doesn’t intersect FF is the empty subgraph. So let’s work out what that means.

For c=1c = 1, there are only two subgraphs of hom(,1)\hom(-, 1), the empty graph and hom(,1)\hom(-, 1). The only dense subobject is F=hom(,1)F = \hom(-, 1); there are no other dense subobjects. So this case is uninteresting and we can ignore it.

For c=2c = 2, there are again precious few subgraphs of hom(,2)\hom(-, 2). [Maybe take a moment to draw this graph.] For example, if a subgraph contains either of the two edges of hom(,2)\hom(-, 2), the CC-action can be used to generate all of hom(,2)\hom(-, 2). So the only proper subgraphs are the discrete ones (the ones with no edges), and the only one of those which is dense is the one with both vertices of hom(,2)\hom(-, 2), namely the subobject

s,t:hom(,1)+hom(,1)hom(,2)\langle s, t \rangle: \hom(-, 1) + \hom(-, 1) \to \hom(-, 2)

which names those two vertices s,ts, t. So F=hom(,1)+hom(,1)F = \hom(-, 1) + \hom(-, 1).

Thus, for c=2c = 2, the induced map

X(2)Set C op(hom(,2),X)Set C op(hom(,1)+hom(,1),X)X(1)×X(1)X(2) \cong Set^{C^{op}}(\hom(-, 2), X) \to Set^{C^{op}}(\hom(-, 1) + \hom(-, 1), X) \cong X(1) \times X(1)

is required to be monic, and this is the only condition imposed by being separated as a presheaf. So we are talking about a subobject X(2)X(1)×X(1)X(2) \hookrightarrow X(1) \times X(1) closed under the symmetry, i.e., a set equipped with a symmetric relation.

You can apply the same type of reasoning to see all of the following categories, in addition to the one above, as quasitoposes of separated ¬¬\neg\neg-presheaves on a smallish category:

  • The category of monomorphisms between sets, as separated presheaves on the interval category 2\mathbf{2}. (You may have heard of \mathcal{M}-categories; these are categories enriched in this quasitopos.)
  • The category of sets equipped with a relation, as separated presheaves on G 1=(0ts1)G_1 = (0 \stackrel{\overset{s}{\to}}{\underset{t}{\to}} 1) (a truncation of the globular category).
  • The category of sets equipped with a reflexive relation, as separated presheaves on a truncated reflexive globular category.
  • The category of sets equipped with a reflexive symmetric relation, as separated presheaves on the full subcategory of finite sets consisting of just the objects 11, 22.
Posted by: Todd Trimble on December 22, 2014 6:00 PM | Permalink | Reply to this

Re: Graph Colouring and Cartesian Closed Categories

People who liked this thread might like

which discusses (among other things) the homomorphism density of graphs GG and HH, meaning the fraction of maps from vertices of GG to vertices of HH that are graph homomorphisms.

It’s a fun short expository paper.

Posted by: John Baez on January 9, 2015 3:18 AM | Permalink | Reply to this

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