# Easy argument for non-equivalent categories

I have to give a talk about some categorical things in a student seminar soon. As this is an introductory talk I cannot assume much knowledge and need very basic arguments.

For example I want to present that the category of sets is not self-dual, i.e. there is no equivalence of categories $$F : Sets \rightarrow Sets^{opp}$$. I will define an equivalence of categories as a fully faithful functor which is also essentially surjective as this is easier to explain in a short amount of time than natural transformations. Could you give me a simple argument? One argument I found online (actually here on stacksexchange) is that $$Sets$$ is a distributive category and $$Sets^{opp}$$ is not but this is too difiicult to explain.

Btw: Does one know a more concrete category that is equivalent to $$Sets^{opp}$$?

I guess this one should be easy enough. Suppose $$F:\textbf{Set} \rightarrow \textbf{Set}^{\text{op}}$$ is an equivalence of categories. Recall, that $$\text{Hom}_{\textbf{Set}}(M,N) = \emptyset$$ iff $$M$$ is non-empty and $$N$$ is empty. Now let $$M$$ be a non-empty set. We get $$\emptyset = \text{Hom}_{\textbf{Set}}(M,\emptyset) \cong_{\textbf{Set}} \text{Hom}_{\textbf{Set}^{\text{op}}}(F(M),F(\emptyset)) = \text{Hom}_{\textbf{Set}}(F(\emptyset),F(M)),$$ such that $$F(M) = \emptyset$$ and $$F(\emptyset) \neq \emptyset$$. Thus $$F$$ is not essentially surjective in contradiction to our assumption.

Yes, the category $$\textbf{Caba}$$ of complete atomic boolean algebras. This is sometimes referred to as the Lindenbaum-Tarski duality and the equivalence is given by the power set functor.

• $F$ is not essentially surjective because no set of cardinality different than $|F(\emptyset)|$ and 0 is of the form $F(X)$? – Josh Jun 18 '19 at 18:34
• Yes, exactly. You got it. – TMO Jun 18 '19 at 18:38
• Thanks a lot for the example, but it probabaly also takes too long to explain the details of your cabas. – Josh Jun 18 '19 at 18:43

$$\textbf{Set}$$ has an initial object (empty set) and a terminal object (any one-element set). There is a morphism from any initial object to any terminal object but no morphism from any terminal object to any initial object.

Dually, $$\textbf{Set}^{\textrm{op}}$$ has an initial object and a terminal object. There is a morphism from any terminal object to any initial object but no morphism from any initial object to any terminal object.

So $$\textbf{Set}$$ and $$\textbf{Set}^{\textrm{op}}$$ cannot be equivalent categories.

Now when you talk about "a more concrete category" do you mean a concrete category? That Wikipedia pages shows how $$\textbf{Set}^{\textrm{op}}$$ is a concrete category.

• No, I just wanted an example of a category that is equivalent to $Sets^{opp}$ to show how we can understand $Sets^{opp}$ in a more concrete way without just reversing arrows. – Josh Jun 18 '19 at 18:36
• Your argument is essentially the same as the one of @ThorWittich right? Just for singletons? – Josh Jun 18 '19 at 18:44