nLab final functor

Idea

A functor $F : C \to D$ is final, if we can restrict diagrams on $D$ to diagrams on $C$ along $F$ without changing their colimit.

Dually, a functor is initial if pulling back diagrams along it does not change the limits of these diagrams.

Beware that this property is pretty much unrelated to that of a functor being an initial object or terminal object in the functor category $[C,D]$. The terminology comes instead from the fact that an object $d\in D$ is initial (resp. terminal) just when the corresponding functor $d:1\to D$ is initial (resp. final).

Remark

(warning on terminology)
In older references (and also some others like HTT), final functors are sometimes called cofinal, the terminology having been imported from order theory (e.g. cofinality). However, this is confusing in category theory because usually the prefix “co-” denotes dualization. In at least one place (Borceux) this non-dualization was treated as a dualization and the word “final” used for the dual concept, but in general it seems that the consensus is to use “final” for what used to be called “cofinal”, and “initial” for the dual concept (since “co-final” would be ambiguous). For example, Johnstone in Sketches of an Elephant before Proposition B2.5.12 says: : Traditionally, final functors were called ‘cofinal functors’; but this use of ‘co’ is potentially misleading as it has nothing to do with dualization — it is derived from the Latin ‘cum’ rather than ‘contra’ — and so it is now generally omitted.

See also the warning at final $(\infty,1)$-functor (here).

Definition

Definition

A functor $F : C \to D$ is final if for every object $d \in D$ the comma category $(d/F)$ is (non-empty and) connected (the non-emptiness condition is redundant since connected categories are non-empty by convention).

A functor $F : C \to D$ is initial if the opposite $F^{op} : C^{op} \to D^{op}$ is final, i.e. if for every object $d \in D$ the comma category $(F/d)$ is connected.

Properties

Proposition

Let $F : C \to D$ be a functor

The following conditions are equivalent.

1. $F$ is final.

2. For all functors $G : D \to Set$ the natural function between colimits

   $$\lim_\to G \circ F \to \lim_{\to} G$$

   is a bijection.

3. For all categories $E$ and all functors $G : D \to E$ the natural morphism between colimits

   $$\lim_\to G \circ F \to \lim_{\to} G$$

   is a isomorphism.

4. For all functors $G : D^{op} \to Set$ the natural function between limits

   $$\lim_\leftarrow G \to \lim_\leftarrow G \circ F^{op}$$

   is a bijection.

5. For all categories $E$ and all functors $G : D^{op} \to E$ the natural morphism

   $$\lim_\leftarrow G \to \lim_\leftarrow G \circ F^{op}$$

   is an isomorphism.

6. For all $d \in D$

   $${\lim_\to}_{c \in C} Hom_D(d,F(c)) \simeq * \,.$$

Proposition

If $F : C \to D$ is final then $C$ is connected precisely if $D$ is.

Proposition

If $F_1$ and $F_2$ are final, then so is their composite $F_1 \circ F_2$.

If $F_2$ and the composite $F_1 \circ F_2$ are final, then so is $F_1$.

If $F_1$ is a full and faithful functor and the composite is final, then both functors seperately are final.

The first two statements of Proposition in fact follow from the stability properties of orthogonal factorization systems:

Proposition

Final functors and discrete fibrations form an orthogonal factorization system called the comprehensive factorization system.

Generalizations

The generalization of the notion of final functor from category theory to (∞,1)-higher category theory is described at

• final (∞,1)-functor.

The characterization of final functors is also a special case of the characterization of exact squares.

Examples

Example

(inclusion of a terminal object is final functor)
If $D$ has a terminal object then the functor $F : {*} \to D$ that picks that terminal object is final: for every $d \in D$ the comma category $d/F$ is equivalent to $*$. The converse is also true: if a functor $*\to D$ is final, then its image is a terminal object.

In this case the statement about preservation of colimits states that the colimit over a category with a terminal object is the value of the diagram at that object. Which is also readily checked directly.

Example

Every right adjoint functor is final.

Proof

Let $(L \dashv R) : C \to D$ be a pair of adjoint functors.To see that $R$ is final, we may for instance check that for all $d \in D$ the comma category $d / R$ is non-empty and connected:

It is non-empty because it contains the adjunction unit $(L(d), d \to R L (d))$. Similarly, for

$$\array{ && d \\ & {}^{\mathllap{f}}\swarrow && \searrow^{\mathrlap{g}} \\ R(a) &&&& R(b) }$$

two objects, they are connected by a zig-zag going through the unit, by the universal factorization property of adjunctions

$$\array{ && d \\ & \swarrow &\downarrow& \searrow \\ R(a) &\stackrel{R \bar f}{\leftarrow}& R L (d)& \stackrel{R(\bar g)}{\to} & R(b) } \,.$$

Example

The inclusion $\mathcal{C} \to \tilde \mathcal{C}$ of any category into its idempotent completion is final.

See at idempotent completion in the section on Finality.

Example

The inclusion of the cospan diagram into its cocone

$$\left( \array{ a \\ \downarrow \\ c \\ \uparrow \\ b } \right) \hookrightarrow \left( \array{ a \\ \downarrow & \searrow \\ c &\longrightarrow & p \\ \uparrow & \nearrow \\ b } \right)$$

is initial.

Remark

By the characterization (here) of limits in a slice category, this implies that fiber products in a slice category are computed as fiber products in the underlying category, or in other words that dependent sum to the point preserves fiber products.

Example

For $\Delta^{op}$ the opposite of the simplex category, the non-full subcategory inclusion of the lowest two face maps

$$\big( [1] \underoverset {d_0} {d_1} {\rightrightarrows} [0] \big) \;\xrightarrow{\;\;\;\;}\; \Delta^{op}$$

is a final functor.

It follows that the colimit over a simplicial diagram is equivalently the coequalizer of the lowest two face maps.

(e.g. Riehl 14, Exp. 8.3.8)

Related concepts

• final functor, cofinal diagram

• homotopy final functor

• final (∞,1)-functor

References

• Masaki Kashiwara, Pierre Schapira, Section 2.5 of: Categories and Sheaves

• Saunders Mac Lane, Section IX.3 of: Categories for the Working Mathematician

• Francis Borceux, Section 2.11 of: Handbook of Categorical Algebra 1, Basic category theory, Encyclopedia of Mathematics and its Applications 50, Cambridge University Press (1994)

  (this says “final functor” for the version under which limits are invariant)

• Emily Riehl, Section 8.3 of: Categorical Homotopy Theory, Cambridge University Press, 2014 (pdf, doi:10.1017/CBO9781107261457)

• Paolo Perrone, Walter Tholen, Kan extensions are partial colimits, Applied Categorical Structures, 2022. (arXiv:2101.04531)

  (this says “confinal functor” for the version under which colimits are invariant)