nLab initial object

Redirected from "coterminal object".
Contents

Context

Category theory

Limits and colimits

Contents

1. Definition

Definition 1.1. An initial object in a category ๐’ž\mathcal{C} is an object โˆ…\emptyset such that for all objects xโˆˆ๐’žx \,\in\, \mathcal{C}, there is a unique morphism โˆ…โ†’โˆƒ!x\varnothing \xrightarrow{\exists !} x with source โˆ…\varnothing. and target xx.

Remark 1.2. An initial object, if it exists, is unique up to unique isomorphism, so that we may speak of the initial object.

Remark 1.3. When it exists, the initial object is the colimit over the empty diagram.

Remark 1.4. Initial objects are also called coterminal, and (rarely, though): coterminators, universal initial, co-universal, or simply universal.

Definition 1.5. An initial object โˆ…\varnothing is called a strict initial object if all morphisms xโ†’โˆ…x \xrightarrow{\;} \varnothing into it are isomorphisms.

Remark 1.6. Initial objects are the dual concept to terminal objects: an initial object in CC is the same as a terminal object in the opposite category C opC^{op}.

Remark 1.7. An object that is both initial and terminal is called a zero object.

2. Examples

3. Properties

Left adjoints to constant functors

Proposition 3.1. Let ๐’ž\mathcal{C} be a category.

  1. The following are equivalent:

    1. ๐’ž\mathcal{C} has a terminal object;

    2. the unique functor ๐’žโ†’*\mathcal{C} \to \ast to the terminal category has a right adjoint

      *โŠฅโŸถโŸต๐’ž \ast \underoverset {\underset{}{\longrightarrow}} {\overset{}{\longleftarrow}} {\bot} \mathcal{C}

    Under this equivalence, the terminal object is identified with the image under the right adjoint of the unique object of the terminal category.

  2. Dually, the following are equivalent:

    1. ๐’ž\mathcal{C} has an initial object;

    2. the unique functor ๐’žโ†’*\mathcal{C} \to \ast to the terminal category has a left adjoint

      ๐’žโŠฅโŸถโŸต* \mathcal{C} \underoverset {\underset{}{\longrightarrow}} {\overset{}{\longleftarrow}} {\bot} \ast

    Under this equivalence, the initial object is identified with the image under the left adjoint of the unique object of the terminal category.

Proof. Since the unique hom-set in the terminal category is the singleton, the hom-isomorphism characterizing the adjoint functors is directly the universal property of an initial object in ๐’ž\mathcal{C}

Hom ๐’ž(L(*),X)โ‰ƒHom *(*,R(X))=* Hom_{\mathcal{C}}( L(\ast) , X ) \;\simeq\; Hom_{\ast}( \ast, R(X) ) = \ast

or of a terminal object

Hom ๐’ž(X,R(*))โ‰ƒHom *(L(X),*)=*, Hom_{\mathcal{C}}( X , R(\ast) ) \;\simeq\; Hom_{\ast}( L(X), \ast ) = \ast \,,

respectively.ย ย โ–ฎ

Cones over the identity

By definition, an initial object is equipped with a universal cocone under the unique functor โˆ…โ†’C\emptyset\to C from the empty category. On the other hand, if II is initial, the unique morphisms !:Iโ†’x!: I \to x form a cone over the identity functor, i.e. a natural transformation ฮ”Iโ†’Id C\Delta I \to Id_C from the constant functor at the initial object to the identity functor. In fact this is almost another characterization of an initial object (e.g. MacLane, p. 229-230):

Lemma 3.2. Suppose IโˆˆCI\in C is an object equipped with a natural transformation p:ฮ”Iโ†’Id Cp:\Delta I \to Id_C such that p I=1 I:Iโ†’Ip_I = 1_I : I\to I. Then II is an initial object of CC.

Proof. Obviously II has at least one morphism to every other object XโˆˆCX\in C, namely p Xp_X, so it suffices to show that any f:Iโ†’Xf:I\to X must be equal to p Xp_X. But the naturality of pp implies that Id C(f)โˆ˜p I=p Xโˆ˜ฮ” I(f)\Id_C(f) \circ p_I = p_X \circ \Delta_I(f), and since p I=1 Ip_I = 1_I this is to say fโˆ˜1 I=p Xโˆ˜1 If \circ 1_I = p_X \circ 1_I, i.e. f=p Xf=p_X as desired.ย ย โ–ฎ

Theorem 3.3. An object II in a category CC is initial iff II is the limit of the identity functor Id CId_C.

Proof. If II is initial, then there is a cone (! X:Iโ†’X) XโˆˆOb(C)(!_X: I \to X)_{X \in Ob(C)} from II to Id CId_C. If (p X:Aโ†’X) XโˆˆOb(C)(p_X: A \to X)_{X \in Ob(C)} is any cone from AA to Id CId_C, then p X=fโˆ˜p Yp_X = f \circ p_Y for any f:Yโ†’Xf:Y\to X, and so in particular p X=! Xโˆ˜p Ip_X = !_X \circ p_I. Since this is true for any XX, p I:Aโ†’Ip_I: A \to I defines a morphism of cones, and it is the unique morphism of cones since if qq is any morphism of cones, then p I=! Iโˆ˜q=1 Iโˆ˜q=qp_I = !_I \circ q = 1_I \circ q = q (using that ! I=1 I!_I = 1_I by initiality). Thus (! X:Iโ†’X) XโˆˆOb(C)(!_X: I \to X)_{X \in Ob(C)} is the limit cone.

Conversely, if (p X:Lโ†’X) XโˆˆOb(C)(p_X: L \to X)_{X \in Ob(C)} is a limit cone for Id CId_C, then fโˆ˜p Y=p Xf\circ p_Y = p_X for any f:Yโ†’Xf:Y\to X, and so in particular p Xโˆ˜p L=p Xp_X \circ p_L = p_X for all XX. This means that both p L:Lโ†’Lp_L: L \to L and 1 L:Lโ†’L1_L: L \to L define morphisms of cones; since the limit cone is the terminal cone, we infer p L=1 Lp_L = 1_L. Then by Lemma 3.2 we conclude LL is initial.ย ย โ–ฎ

Remark 3.4. (relevance for adjoint functor theorem)

Theorem 3.3 is actually a key of entry into the general adjoint functor theorem. Showing that a functor G:Cโ†’DG: C \to D has a left adjoint is tantamount to showing that each functor D(d,Gโˆ’)D(d, G-) is representable, i.e., that the comma category dโ†“Gd \downarrow G has an initial object (c,ฮธ:dโ†’Gc)(c, \theta: d \to G c) (see at adjoint functor, this prop.). This is the limit of the identity functor, but typically this is the limit over a large diagram whose existence is not guaranteed. The point of a solution set condition is to replace this with a small diagram which is cofinal in the large diagram.

5. References

Textbook accounts:

Last revised on October 29, 2024 at 10:51:35. See the history of this page for a list of all contributions to it.