In mathematics, specifically category theory, an overcategory (and undercategory) is a distinguished class of categories used in multiple contexts, such as with covering spaces (espace etale). They were introduced as a mechanism for keeping track of data surrounding a fixed object ${\displaystyle X}$ in some category ${\displaystyle {\mathcal {C))}$. There is a dual notion of undercategory, which is defined similarly.

Definition

Let ${\displaystyle {\mathcal {C))}$ be a category and ${\displaystyle X}$ a fixed object of ${\displaystyle {\mathcal {C))}$^{[1]pg 59}. The overcategory (also called a slice category) ${\displaystyle {\mathcal {C))/X}$ is an associated category whose objects are pairs ${\displaystyle (A,\pi )}$ where ${\displaystyle \pi :A\to X}$ is a morphism in ${\displaystyle {\mathcal {C))}$. Then, a morphism between objects ${\displaystyle f:(A,\pi )\to (A',\pi ')}$ is given by a morphism ${\displaystyle f:A\to A'}$ in the category ${\displaystyle {\mathcal {C))}$ such that the following diagram commutes

${\displaystyle {\begin{matrix}A&\xrightarrow {f} &A'\\\pi \downarrow {\text{ ))&{\text{ ))&{\text{ ))\downarrow \pi '\\X&=&X\end{matrix))}$

There is a dual notion called the undercategory (also called a coslice category) ${\displaystyle X/{\mathcal {C))}$ whose objects are pairs ${\displaystyle (B,\psi )}$ where ${\displaystyle \psi :X\to B}$ is a morphism in ${\displaystyle {\mathcal {C))}$. Then, morphisms in ${\displaystyle X/{\mathcal {C))}$ are given by morphisms ${\displaystyle g:B\to B'}$ in ${\displaystyle {\mathcal {C))}$ such that the following diagram commutes

${\displaystyle {\begin{matrix}X&=&X\\\psi \downarrow {\text{ ))&{\text{ ))&{\text{ ))\downarrow \psi '\\B&\xrightarrow {g} &B'\end{matrix))}$

These two notions have generalizations in 2-category theory^[2] and higher category theory^{[3]pg 43}, with definitions either analogous or essentially the same.

Properties

Many categorical properties of ${\displaystyle {\mathcal {C))}$ are inherited by the associated over and undercategories for an object ${\displaystyle X}$. For example, if ${\displaystyle {\mathcal {C))}$ has finite products and coproducts, it is immediate the categories ${\displaystyle {\mathcal {C))/X}$ and ${\displaystyle X/{\mathcal {C))}$ have these properties since the product and coproduct can be constructed in ${\displaystyle {\mathcal {C))}$, and through universal properties, there exists a unique morphism either to ${\displaystyle X}$ or from ${\displaystyle X}$. In addition, this applies to limits and colimits as well.

Examples

Overcategories on a site

Recall that a site ${\displaystyle {\mathcal {C))}$ is a categorical generalization of a topological space first introduced by Grothendieck. One of the canonical examples comes directly from topology, where the category ${\displaystyle {\text{Open))(X)}$ whose objects are open subsets ${\displaystyle U}$ of some topological space ${\displaystyle X}$, and the morphisms are given by inclusion maps. Then, for a fixed open subset ${\displaystyle U}$, the overcategory ${\displaystyle {\text{Open))(X)/U}$ is canonically equivalent to the category ${\displaystyle {\text{Open))(U)}$ for the induced topology on ${\displaystyle U\subseteq X}$. This is because every object in ${\displaystyle {\text{Open))(X)/U}$ is an open subset ${\displaystyle V}$ contained in ${\displaystyle U}$.

Category of algebras as an undercategory

The category of commutative ${\displaystyle A}$-algebras is equivalent to the undercategory ${\displaystyle A/{\text{CRing))}$ for the category of commutative rings. This is because the structure of an ${\displaystyle A}$-algebra on a commutative ring ${\displaystyle B}$ is directly encoded by a ring morphism ${\displaystyle A\to B}$. If we consider the opposite category, it is an overcategory of affine schemes, ${\displaystyle {\text{Aff))/{\text{Spec))(A)}$, or just ${\displaystyle {\text{Aff))_{A))$.

Overcategories of spaces

Another common overcategory considered in the literature are overcategories of spaces, such as schemes, smooth manifolds, or topological spaces. These categories encode objects relative to a fixed object, such as the category of schemes over ${\displaystyle S}$, ${\displaystyle {\text{Sch))/S}$. Fiber products in these categories can be considered intersections, given the objects are subobjects of the fixed object.