A functor $F \colon C \to D$ is final, if restricting diagrams along $F$ does not change 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 $d \colon 1\to D$ is initial (resp. final).
(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 (cf. 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 says (before Proposition B2.5.12 ):
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).
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.
Let $F : C \to D$ be a functor
The following conditions are equivalent.
$F$ is final.
For all functors $G : D \to Set$ the natural function between colimits
is a bijection.
For all categories $E$ and all functors $G : D \to E$ the natural morphism between colimits
is an isomorphism.
For all functors $G : D^{op} \to Set$ the natural function between limits
is a bijection.
For all categories $E$ and all functors $G : D^{op} \to E$ the natural morphism
is an isomorphism.
For all $d \in D$
for all functors $G: D \to E$, the canonical map
between the functors of cocones on $G$ and $GF$ is an isomorphism. (Here, the $\Delta$‘s denote the functors sending objects to constant functors, and $Nat$ stands for the set of natural transformations.)
(Kashiwara & Schapira 2006, Prop. 2.5.2)
If $F : C \to D$ is final then $C$ is connected precisely if $D$ is.
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 $F_1$.
Final functors are stable under pushout.
The coproduct of final functors in the arrow category $\mathbf{Cat}^{\mathbf{2}}$ is a final functor.
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:
Final functors and discrete fibrations form an orthogonal factorization system called the comprehensive factorization system.
The characterization of final functors is a special case of the characterization of exact squares.
Finality for enriched functors with respect to weighted colimits (for enrichment in a cartesian closed category) is discussed in Kelly 1982 §4.5: see Theorem 4.67 in particular.
See also discussion of finality specifically for coends (initiality for weighted limits and ends) by:
The generalization of the notion of final functor from category theory to (∞,1)-higher category theory is described at
(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.
More generally, a functor whose domain is a discrete category is final if and only if it is a right adjoint.
A functor between groupoids is final iff it is essentially surjective and full.
Every right adjoint functor is final.
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
two objects, they are connected by a zig-zag going through the unit, by the universal factorization property of adjunctions
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.
The inclusion of the cospan diagram into its cocone
is initial.
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.
For $\Delta^{op}$ the opposite of the simplex category, the non-full subcategory inclusion of the lowest two face maps
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)
basic properties of…
Masaki Kashiwara, Pierre Schapira, Section 2.5 of: Categories and Sheaves, Grundlehren der Mathematischen Wissenschaften 332, Springer (2006) [doi:10.1007/3-540-27950-4, pdf]
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)
In internal category theory
for internal functors between internal groupoids in exact categories:
Last revised on July 19, 2024 at 14:15:19. See the history of this page for a list of all contributions to it.