## February 13, 2008

### 2-Galois and 2-Logic

#### Posted by David Corfield

Let’s boldly venture on with our ascent of Mount 2-Logic.

Todd told us about Galois connections via relations

any relation $R \subseteq A \times B$ whatsoever can be used to set up a Galois connection between subsets $A'$ of $A$ and subsets $B'$ of $B$: $\frac{A' \subseteq \{a \in A: \forall_{b: B'} (a, b) \in R\} }{B' \subseteq \{b \in B: \forall_{a: A'} (a, b) \in R\} } iff$

And he also told us that the Galois connection relating theories to symmetry groups works by way of a relation:

In our case, $A$ is the group of permutations $g$ on $X$, $B$ is the set of finitary relations $p$ on $X$, and the relation $R$ is the set of $(g, p)$ such that $p(x_1, \ldots, x_n) = p(g x_1, \ldots, g x_n)$ [for all $n$-tuples $(x_1, \ldots, x_n)$ if $p$ is $n$-ary].

Let me try to get this straight. We can upgrade to a Galois correspondence if we take only the fixed points of the adjunction. These are subgroups of $G$ on one side, and subtheories of $P(X^*)$ on the other. The closure operators send collections of predicates to the their ‘theory’ closure, and collections of elements of $G$ to the subset of $G$ they generate.

Up the ladder we might hope for a (2)-connection which improves to a (2)-correspondence between 2-groups and 2-theories. So let’s recall what happens with a categorified connection:

each 2-relation $R: A^{op} \times B^{op} \to Set$ induces a pair of contravariant functors

$\frac{A^{'}(?) \to hom_{PB}(B^{'}(?),R(?, -))}{B^{'}(-) \to hom_{PA}(A^{'}(-),R(?,-))}$

[Just to help myself, I tried to think this out with $A$ and $B$ discrete. You get something like an isomorphism between

$\product_i (\product_j r_{ij}^{b_j})^{a_i}$ and $\product_j (\product_i r_{ij}^{a_i})^{b_j}.$

Apologies to any Australians reading for this horrible piece of concreteness.]

So where can we find a willing 2-relation to categorify the relation $R$, the set of $(g, p)$ such that $p(x_1, \ldots, x_n) = p(g x_1, \ldots, g x_n)$, all $n$-tuples $(x_1, \ldots, x_n)$?

In a single-sorted 2-theory we’ll have a category $C$ (or maybe a groupoid) instead of $X$, and then $A$ will presumably become the 2-group of automorphisms of $C$.

$B$ can become a category of functors from $C^n$ to Set for different values of $n$. So the sum over $n$ of the category of presheaves on $C^n$.

Then $R$ will need to be a functor from $A \times B$ (or perhaps their ops) to Set. So how will $R$ act on $(g, p)$, $g \in Aut(C)$ and $p: C^n \to Set$, to yield a set?

There’s plenty to remind one here of things glimpsed earlier – Kleinian 2-geometry, etc. – but my poor brain needs a rest right now.

Posted at February 13, 2008 9:44 PM UTC

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### Re: 2-Galois and 2-Logic

$(g, p) \in R iff p(x_1, \ldots, x_n) = p(g x_1, \ldots, g x_n), all (x_1, \ldots, x_n),$

we could define an action of $G$ on $P(X^n)$, so that $g \cdot p(x_1, \ldots, x_n) = p(g x_1, \ldots, g x_n)$.

Then

$(g, p) \in R iff g \cdot p = p.$

Up one level, we have $G = AUT(C)$ acting on $P(C^n)$. Now why not define

$R(g, p) = Isom_{P(C^n)}(p, g \cdot p),$

a set of isomorphisms of presheaves?

Posted by: David Corfield on February 14, 2008 11:21 AM | Permalink | Reply to this

### Re: 2-Galois and 2-Logic

Wow. Now ‘categorified connection’ means two really different things on this blog!

(If anyone explains how Galois connections and connections on bundles are special cases of a more general concept, I’ll kill ‘em.)

Posted by: John Baez on February 14, 2008 9:25 PM | Permalink | Reply to this

### Re: 2-Galois and 2-Logic

A long time ago (in an old Klein 2-geometry thread) we were having trouble deciding how to define a sub-2-group. Could it be that this was because we didn’t think back then about how to categorify the power set construction?

Todd has told us that people use $P C$ to refer to $Set^{C^{op}}$, for a category $C$, just as we use $P X$ to refer to $2^X$, for a set $X$.

If a subgroup of $G$ is a kind of member of $P U G$, the power set of the underlying set of $G$, should a sub-2-group of a 2-group be a kind of presheaf on the underlying category of the 2-group?

Then again, we might have told a similar story already for groups themselves, which are after all categories. So that, as Lawvere said, a presheaf on a group is a predicate.

Posted by: David Corfield on February 18, 2008 9:31 AM | Permalink | Reply to this

### Re: 2-Galois and 2-Logic

Since a 2-group is a groupoid, there is a simplification in the classification of subobjects : there is an equivalence between presheafs $UG\to \mathrm{Set}$ and faithful functors $C\to UG$, where $C$ is a groupoid. So the subobjects given by predicates with “truth value” in $\mathrm{Set}$ are the faithful functors into $G$ (in the 2-category of groupoids).

But you can still classify subobjects with the old classifier $1\to\Omega$ (where $\Omega$ is the category of subsets of $1$, i.e. the order of truth values*). There is an equivalence between functors $UG\to \Omega$ (i.e presheafs which take value in $\Omega\hookrightarrow\mathrm{Set}$) and full and faithful functors $C\to UG$, where $C$ is a groupoid. So the subobjects corresponding to predicates with truth value in $\Omega$ are the full and faithful functors.

* I don’t call it $2$ because I prefer not to suppose that $\mathrm{Set}$ is boolean, but it’s another question.

Posted by: Mathieu Dupont on February 18, 2008 12:19 PM | Permalink | Reply to this

### Re: 2-Galois and 2-Logic

Maybe, as John once said, we’ll end up with split forms of categorified notions such as sub-2-groups, depending on use.

Posted by: David Corfield on February 19, 2008 6:37 PM | Permalink | Reply to this

### Re: 2-Galois and 2-Logic

How does the Galois theory of deck transformations and covering spaces fit in with what’s being discussed here.

In the Trimble-Dolan setting, we have a set $X$ and we’re going to put into correspondence single-sorted $X$-theories, and subgroups of $X !$, permutations of $X$.

You might at first hope that unary predicates would be enough to pin down the set members sufficiently to restrict allowable symmetries to a given subset of $X !$. But of course this isn’t so. If $X = \{a, b, c\}$, then unary predicates aren’t enough for a theory invariant under $A_3$, even permutations. Binary predicates, however, are enough.

[Is there a general result as to how small the largest $n$ need be such that all subgroups of $m !$ are captured by theories with predicates of arity no more than $n$?]

So, here we pack together all powers of $X$, and then look at collections of maps to $2$, or truth-valued fibrations above it.

When we turn to covering spaces, then, say of the circle, why aren’t we seeing powers of the circle, just as we saw powers of $X$? Perhaps it’s because this situation is already 2-Galois. After all, we probe this circle by looking at locally constant sheaves on it, which seems to be a rather like we’re treating it as a category.

Say we start with the fundamental groupoid of the circle, $\Pi_1 (S^1)$, which has a 2-group acting on it, $AUT (\Pi_1 (S^1))$. My attempt to find a 2-correspondence was to look to (pre)-sheaves over powers of the former and representations of the latter.

Powers again.

Posted by: David Corfield on February 18, 2008 11:10 AM | Permalink | Reply to this

### Re: 2-Galois and 2-Logic

I seem to be rambling on by myself here, but a couple of thoughts. Were the space not connected I could see we might need locally constant sheaves on powers of the space.

As a reminder, the thought that we ought to have something to do with $P P C$ in 2-Galois relates to the Grothendieckian take on fundamental groups as the symmetries of a map from the category of locally constant sheaves on a space to Set.

Posted by: David Corfield on February 19, 2008 6:45 PM | Permalink | Reply to this

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