The traditional denotation of a structured set object is something like

$(1)\qquad(X,\mathcal S_1,\dots\mathcal S_n)$

for some "structures" $\mathcal S_1,\dots\mathcal S_n$ on a set X.

The modern definition of a category with sets as objects and functions as morphisms includes a faithful functor $F:\mathbf C\to\mathbf{Set}$, that is, a functor such that $\alpha\ne \beta\implies F(\alpha)\ne F(\beta)$ for all morphisms $\alpha,\beta\in Mor(\mathbf C)$.

Now there is a faithful functor from Rel to Set, $\mathcal P(\cdot)$ the covariant power set functor, and my question is:

is it possible to express this category as a category of structured sets as in $(1)$ with structure preserving functions as morphisms?


Well, $\mathbf{Rel}$ is a full subcategory of $\mathbf{CJS}$, the category of complete join-semilattices (and join-preserving maps). The embedding sends a set $X$ to the powerset $\mathscr{P} X$ and a relation $R : X \not\to Y$ to the unique join-preserving map $r : \mathscr{P} X \to \mathscr{P} Y$ where $r \{ x \} = \{ y \in Y : x \mathrel{R} y \}$. Thus you can realise $\mathbf{Rel}$ as a full subcategory of a category of (infinitary) algebraic structures.

  • $\begingroup$ I almost expected something like that. But it would be cool to see an explicit description. $\endgroup$ – Lehs Jan 5 '15 at 14:59
  • $\begingroup$ This is an explicit description. $\endgroup$ – Zhen Lin Jan 5 '15 at 15:13
  • $\begingroup$ Do you mean that $(\mathscr PX,\vee)$ are the objects I asked for? $\endgroup$ – Lehs Jan 5 '15 at 18:28

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