An initial object in a category $C$ is an object $\emptyset$ such that for any object $x$ of $C$, there is a unique morphism $!:\emptyset\to x$. An initial object, if it exists, is unique up to unique isomorphism, so we speak of the initial object.
An initial object may also be called coterminal, universal initial, co-universal, or simply universal.
Initial objects are the dual concept to terminal objects: an initial object in $C$ is the same as a terminal object in $C^{op}$. An object that is both initial and terminal is called a zero object.
Hence the initial object may also be viewed as the colimit over the empty diagram.
An initial object in a poset is a bottom element.
Likewise, the empty category is an initial object in Cat, the empty space is an initial object in Top, and so on.
The trivial group is the initial object (in fact, the zero object) of Grp and Ab.
An initial object $\emptyset$ is called a strict initial object if any morphism $x\to \emptyset$ must be an isomorphism. The initial objects of a poset, of $Set$, $Cat$, $Top$, and of any topos (in fact, any extensive category, or any cartesian closed category) are strict. At the other extreme, a zero object is only a strict initial object if the category is trivial (equivalent to the terminal category).
Let $\mathcal{C}$ be a category.
The following are equivalent:
$\mathcal{C}$ has a terminal object;
the unique functor $\mathcal{C} \to \ast$ to the terminal category has a right adjoint
Under this equivalence, the terminal object is identified with the image under the right adjoint of the unique object of the terminal category.
Dually, the following are equivalent:
$\mathcal{C}$ has an initial object;
the unique functor $\mathcal{C} \to \ast$ to the terminal category has a left adjoint
Under this equivalence, the initial object is identified with the image under the left adjoint of the unique object of the terminal category.
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 $\mathcal{C}$
or of a terminal object
respectively.
By definition, an initial object is equipped with a universal cocone under the unique functor $\emptyset\to C$ from the empty category. On the other hand, if $I$ is initial, the unique morphisms $!: I \to x$ form a cone over the identity functor, i.e. a natural transformation $\Delta I \to Id_C$ from the constant functor at the initial object to the identity functor. In fact this is almost another characterization of an initial object:
Suppose $I\in C$ is an object equipped with a natural transformation $p:\Delta I \to Id_C$ such that $p_I = 1_I : I\to I$. Then $I$ is an initial object of $C$.
Obviously $I$ has at least one morphism to every other object $X\in C$, namely $p_X$, so it suffices to show that any $f:I\to X$ must be equal to $p_X$. But the naturality of $p$ implies that $\Id_C(f) \circ p_I = p_X \circ \Delta_I(f)$, and since $p_I = 1_I$ this is to say $f \circ 1_I = p_X \circ 1_I$, i.e. $f=p_I$ as desired.
An object $I$ is initial in $C$ iff $I$ is the limit of $Id_C$.
If $I$ is initial, then there is a cone $(!_X: I \to X)_{X \in Ob(C)}$ from $I$ to $Id_C$, and if $(p_X: A \to X)_{X \in Ob(C)}$ is any cone from $A$ to $Id_C$, then $p_X = !_X \circ p_I$. Hence $p_I: A \to I$ defines a morphism of cones, and the unique morphism of cones since if $q$ any morphism of cones, then $p_I = !_I \circ q = 1_I \circ q = q$. Thus $(!_X: I \to X)_{X \in Ob(C)}$ is the limit cone.
Conversely, if $(p_X: L \to X)_{X \in Ob(C)}$ is a limit cone for $Id_C$, then $p_X \circ p_L = p_X$ for all $X$. This means that both $p_L: L \to L$ and $1_L: L \to L$ define morphisms of cones; since the limit cone is the terminal cone, we infer $p_L = 1_L$. Then by Theorem 1 we conclude $L$ is initial.
This corollary is actually a key of entry into the general adjoint functor theorem. Showing that a functor $G: C \to D$ has a left adjoint is tantamount to showing that each functor $D(d, G-)$ is representable, i.e., that the comma category $d \downarrow G$ has an initial object $(c, \theta: d \to G c)$. 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.
Last revised on June 12, 2018 at 12:39:27. See the history of this page for a list of all contributions to it.