4.32 Categories over categories
In this section we have a functor p : \mathcal{S} \to \mathcal{C}. We think of \mathcal{S} as being on top and of \mathcal{C} as being at the bottom. To make sure that everybody knows what we are talking about we define the 2-category of categories over \mathcal{C}.
Definition 4.32.1. Let \mathcal{C} be a category. The 2-category of categories over \mathcal{C} is the 2-category defined as follows:
Its objects will be functors p : \mathcal{S} \to \mathcal{C}.
Its 1-morphisms (\mathcal{S}, p) \to (\mathcal{S}', p') will be functors G : \mathcal{S} \to \mathcal{S}' such that p' \circ G = p.
Its 2-morphisms t : G \to H for G, H : (\mathcal{S}, p) \to (\mathcal{S}', p') will be morphisms of functors such that p'(t_ x) = \text{id}_{p(x)} for all x \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{S}).
In this situation we will denote
\mathop{\mathrm{Mor}}\nolimits _{\textit{Cat}/\mathcal{C}}(\mathcal{S}, \mathcal{S}')
the category of 1-morphisms between (\mathcal{S}, p) and (\mathcal{S}', p')
In this 2-category we define horizontal and vertical composition exactly as is done for \textit{Cat} in Section 4.28. The axioms of a 2-category are satisfied for the same reason that the hold in \textit{Cat}. To see this one can also use that the axioms hold in \textit{Cat} and verify things such as “vertical composition of 2-morphisms over \mathcal{C} gives another 2-morphism over \mathcal{C}”. This is clear.
Analogously to the fibre of a map of spaces, we have the notion of a fibre category, and some notions of lifting associated to this situation.
Definition 4.32.2. Let \mathcal{C} be a category. Let p : \mathcal{S} \to \mathcal{C} be a category over \mathcal{C}.
The fibre category over an object U\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}) is the category \mathcal{S}_ U with objects
\mathop{\mathrm{Ob}}\nolimits (\mathcal{S}_ U) = \{ x\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{S}) : p(x) = U\}
and morphisms
\mathop{\mathrm{Mor}}\nolimits _{\mathcal{S}_ U}(x, y) = \{ \phi \in \mathop{\mathrm{Mor}}\nolimits _\mathcal {S}(x, y) : p(\phi ) = \text{id}_ U\} .
A lift of an object U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}) is an object x\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{S}) such that p(x) = U, i.e., x\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{S}_ U). We will also sometime say that x lies over U.
Similarly, a lift of a morphism f : V \to U in \mathcal{C} is a morphism \phi : y \to x in \mathcal{S} such that p(\phi ) = f. We sometimes say that \phi lies over f.
There are some observations we could make here. For example if F : (\mathcal{S}, p) \to (\mathcal{S}', p') is a 1-morphism of categories over \mathcal{C}, then F induces functors of fibre categories F : \mathcal{S}_ U \to \mathcal{S}'_ U. Similarly for 2-morphisms.
Here is the obligatory lemma describing the 2-fibre product in the (2, 1)-category of categories over \mathcal{C}.
Lemma 4.32.3. Let \mathcal{C} be a category. The (2, 1)-category of categories over \mathcal{C} has 2-fibre products. Suppose that F : \mathcal{X} \to \mathcal{S} and G : \mathcal{Y} \to \mathcal{S} are morphisms of categories over \mathcal{C}. An explicit 2-fibre product \mathcal{X} \times _\mathcal {S}\mathcal{Y} is given by the following description
an object of \mathcal{X} \times _\mathcal {S} \mathcal{Y} is a quadruple (U, x, y, f), where U \in \mathop{\mathrm{Ob}}\nolimits (\mathcal{C}), x\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{X}_ U), y\in \mathop{\mathrm{Ob}}\nolimits (\mathcal{Y}_ U), and f : F(x) \to G(y) is an isomorphism in \mathcal{S}_ U,
a morphism (U, x, y, f) \to (U', x', y', f') is given by a pair (a, b), where a : x \to x' is a morphism in \mathcal{X}, and b : y \to y' is a morphism in \mathcal{Y} such that
a and b induce the same morphism U \to U', and
the diagram
\xymatrix{ F(x) \ar[r]^ f \ar[d]^{F(a)} & G(y) \ar[d]^{G(b)} \\ F(x') \ar[r]^{f'} & G(y') }
is commutative.
The functors p : \mathcal{X} \times _\mathcal {S}\mathcal{Y} \to \mathcal{X} and q : \mathcal{X} \times _\mathcal {S}\mathcal{Y} \to \mathcal{Y} are the forgetful functors in this case. The transformation \psi : F \circ p \to G \circ q is given on the object \xi = (U, x, y, f) by \psi _\xi = f : F(p(\xi )) = F(x) \to G(y) = G(q(\xi )).
Proof.
Let us check the universal property: let p_\mathcal {W} : \mathcal{W}\to \mathcal{C} be a category over \mathcal{C}, let X : \mathcal{W} \to \mathcal{X} and Y : \mathcal{W} \to \mathcal{Y} be functors over \mathcal{C}, and let t : F \circ X \to G \circ Y be an isomorphism of functors over \mathcal{C}. The desired functor \gamma : \mathcal{W} \to \mathcal{X} \times _\mathcal {S} \mathcal{Y} is given by W \mapsto (p_\mathcal {W}(W), X(W), Y(W), t_ W). Details omitted; compare with Lemma 4.31.4.
\square
Example 4.32.4. The constructions of 2-fibre products of categories over categories given in Lemma 4.32.3 and of categories in Lemma 4.31.4 (as in Example 4.31.3) produce non-equivalent outputs in general. Namely, let \mathcal{S} be the groupoid category with one object and two arrows, and let \mathcal{X} be the discrete category with one object. Taking the 2-fibre product \mathcal{X} \times _\mathcal {S} \mathcal{X} as categories yields the discrete category with two objects. However, if we view all of these as categories over \mathcal{S}, the 2-fiber product \mathcal{X} \times _\mathcal {S} \mathcal{X} as categories over \mathcal{S} is the discrete category with one object. The difference is that (in the notation of Lemma 4.32.3), we were allowed to choose any comparison isomorphism f in the first situation, but could only choose the identity arrow in the second situation.
Lemma 4.32.5. Let \mathcal{C} be a category. Let f : \mathcal{X} \to \mathcal{S} and g : \mathcal{Y} \to \mathcal{S} be morphisms of categories over \mathcal{C}. For any object U of \mathcal{C} we have the following identity of fibre categories
\left(\mathcal{X} \times _\mathcal {S}\mathcal{Y}\right)_ U = \mathcal{X}_ U \times _{\mathcal{S}_ U} \mathcal{Y}_ U
Proof.
Omitted.
\square
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