# What about replacing $\{0,1\}$ in Stone duality with another finite set?

Basically Stone duality or more general the duality between spatial locales and sober spaces is about enriching the set of morphisms $X \to \{0,1\}$ with an additional structure and then finding appropriate conditions for a map $\mathrm{Hom}(X,\{0,1\}) \to \mathrm{Hom}(Y,\{0,1\})$ to be the pullback of a unique morphism $Y \to X$. For example in the discrete case, we get the following basic set-theoretic observation:

Let $X,Y$ be sets. A map $\mathrm{Hom}(X,\{0,1\}) \to \mathrm{Hom}(Y,\{0,1\})$ is the pullback of a morphism iff it preserves arbitrary suprema and finite infima if we endow $\mathrm{Hom}(X,\{0,1\}) \cong \mathcal{P}(X)$ with the partial order given by inclusion.

Question. Is there a similar statement for the functor $\mathrm{Hom}(-,\{0,...,n\})$, where $n \geq 1$? That is can we endow these sets with additional structure such that we can find a criterion when a map $\mathrm{Hom}(X,\{0,...,n\}) \to \mathrm{Hom}(Y,\{0,...,n\})$ is the pullback of a map $Y \to X$?

First observe that there is a natural isomorphism between this functor and $\mathcal{P}^{[n]}$, which is defined by $\mathcal{P}^{[n]}(X) := \{(A_1,...,A_n) : A_1 \subseteq ... \subseteq A_n \subseteq X\}$ on objects and by preimages on morphisms. It is a partial order (inclusion in each entry) with suprema and infima. But it also has various operations (endofunctors) coming from automorphisms of $\{0,...,n\}$. For example, the permutation $i \mapsto n - i$ yields the operation $K : (A_1,....,A_n) \mapsto (A_n^c,...,A_1^c)$ and the cycle $(0 ~ 1)$ yields the operation $H : (A_1,...,A_n) \to (A_2 \setminus A_1,A_2,...,A_n)$.

For $n = 2$ I can show that a map $\mathcal{P}^{[n]}(X) \to \mathcal{P}^{[n]}(Y)$ is a pullback iff it preserves suprema, finite infima and commutes with $H$ and $K$ (for $n=1$ this is also true, we don't even need $H$ or $K$). Remark that $H^2=K^2=(HK)^3=1$ and that $K$ transforms suprema into infima and vice versa. Is this a known structure? If not, it is possible to endow $\mathrm{Hom}(X,3)$ with a more well-known structure?

For $n>2$ I don't think that $H$ and $K$ suffice. But I guess that the whole set of endofunctors of $\mathcal{P}^{[n]}$ suffices.

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Try looking at the edit to Todd's answer to mathoverflow.net/questions/69086/…. It may be relevant. –  Benjamin Steinberg Jan 3 '12 at 14:06
I've looked at it, I think it is not related. –  Martin Brandenburg Jan 3 '12 at 14:35
When I see this, I think "Priestly duality". However, I can't recall the topic or some of the recent workers in it, except that one of them (Brian Davey?) used to write songs about the conferences he attended. I suspect the path you suggest has been trodden. Gerhard "Ask Me Not About Dualities" Paseman, 2012.01.03 –  Gerhard Paseman Jan 3 '12 at 14:51
There is a book about natural dualities in universal algebra. –  Benjamin Steinberg Jan 3 '12 at 20:24
Do you mean "Natural dualities for the working algebraist" by Clark and Davey? –  Martin Brandenburg Jan 3 '12 at 20:51