ct.category theory - Explicit description of a fibered category

This construction may not be the most natural (or general) one, but I find it reasonably enlightening.

Let $mathcal F$ and $mathcal C$ denote the categories with one object associated to $G$ and $H$, respectively. Notice that if $mathcal F'$ is any category equivalent to $mathcal F$, then in particular it admits a fully faithful functor to $mathcal F$. Since $mathcal F$ has only one object, with isomorphisms in bijection with $G$, this implies that every hom-set in $mathcal F'$ must be in bijection with $G$ as well. It's easy to check that every morphism in $mathcal F'$ must be an isomorphism, so this proves that $mathcal F'$ is a groupoid, with exactly $|G|$ isomorphisms between any two objects.

I claim that we can choose $mathcal F'$ to have objects indexed by $H$. To be explicit, let's say that the morphisms between any two objects $h_1, h_2$ are identified with $G$, and that the composition of $g_1: h_1 to h_2$ and $g_2: h_2 to h_3$ is $g_1 g_2: h_1 to h_3$. Then this admits a natural "projection" functor to $mathcal F$, by sending every object to the unique object $*$ of $mathcal F$ and sending each morphism to the morphism of the same name. We get a functor in the other direction by sending $*$ to the object labeled by the identity of $H$, and preserving names of morphisms. The composition $mathcal F to mathcal F' to mathcal F$ is literally the identity functor, and $mathcal F' to mathcal F to mathcal F'$ is easily seen to be naturally isomorphic to the identity functor via a base-preserving natural transformation. So $mathcal F'$ and $mathcal F$ are equivalent fibered categories over $mathcal C$.

Now let's construct a splitting of $mathcal F' to mathcal C$. Fix a representative $widetilde h in G$ for each element $h in H$. Consider the subcategory of $mathcal F'$ that includes all objects, but only the morphisms of the form $widetilde h_1 widetilde h_2^{-1}: h_1 to h_2$. (Note that this does contain identities and compositions.) For any given morphism $h$ in $mathcal C$ and any object $h_2 in mathcal F'$, our chosen subcategory contains a unique pullback $h_1 to h_2$ of $h$, namely the morphism $widetilde h_1 widetilde h_2^{-1}: h_1 to h_2$ with $h_1$ chosen so that $h_1 h_2^{-1} = h$.

To make this more concrete, let's look at the simplest possible non-split group extension: $mathbb Z/4mathbb Z twoheadrightarrow mathbb Z/2mathbb Z$. Here, the category $mathcal F'$ has two objects, with four morphisms between any pair, all of them isomorphisms. This category deserves to be equivalent to $mathcal F$: it has two objects, which both look exactly like the object of $mathcal F$ and are isomorphic to each other. Make $mathcal F'$ into a fibered category over $mathcal C$ by composing the "projection" functor to $mathcal F$ with the given functor $mathcal F to mathcal C$. Recall that we can't construct a splitting of the original fibered category $mathcal F to mathcal C$ precisely because we would need to choose a lift of the morphism $1 in mathbb Z/2mathbb Z$ to $mathbb Z/4mathbb Z$, and neither of the two choices gives something that respects composition. But in our new fibered category $mathcal F'$, we need to choose a lift of $1 in mathbb Z/2mathbb Z$ to some morphism between the two objects of $mathcal F'$, instead of an automorphism of one of the objects. So we don't need to worry about composing the lift with itself, and the problem is avoided.

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