limit of a functor

Let $G$ be a functor from categories $\mathcal{I}$ to $\mathcal{C}$ . A limit of $G$ is a pair $(L,\tau)$ where

1. $L:\mathcal{I}\to \mathcal{C}$ is a constant functor,
2. $\tau: L\to G$ is a natural transformation,

such that it is universal among all pairs satisfying (1) and (2). In other words, for any pair $(M,\varphi)$ , where $M:\mathcal{I}\to \mathcal{C}$ is a constant functor, and $\varphi: M\to G$ is a natural transformation, there is a unique natural transformation $\phi:M\to L$ with the following commutative diagram:

Since we may identify a constant functor with its value, say an object $A$ , in $\mathcal{C}$ , a limit of $G$ may be viewed as an object $A$ in $\mathcal{C}$ , together with a collection of morphisms $A \to G(I)$ , or $G_I$ , for each object $I$ in $\mathcal{I}$ such that

Furthermore, if another object $B$ in $\mathcal{C}$ satisfies $(*)$ , then there is a unique morphism $B\to A$ such that $$B\to A\to G_I = B\to G_I$$ for all objects $I$ in $\mathcal{I}$ .

Remarks.

• A limit of a functor may or may not exist. If it exists, then it is unique up to natural equivalence. In other words, we may call it the limit of $G$ .
• In the literature, the limit of $G$ is variously known as the inverse limit, left limit, projective limit, root, or left root of $G$ . and is generally written $\liminv G$ .
• The category $\mathcal{I}$ above is usually called the index category, and the functor $G$ a diagram in $\mathcal{C}$ .
• The most common index categories are finite categories, discrete categories, partially ordered categories, and more specifically, directed categories.
• On the other hand, it turns out that if, in a category, if equalizers of any pairs of morphisms, and arbitrary products of any collections of objects exist, then it can be shown that every functor into this category has a limit.

By reversing all the arrows in $\mathcal{C}$ , we arrive at the dual concept of a limit, that of a colimit. Precisely, the colimit of $G:\mathcal{I}\to \mathcal{C}$ is a pair $(R,\tau)$ where $R:\mathcal{I}\to \mathcal{C}$ is a constant functor and $\tau: G\to R$ is a natural transformation, such that for any pair $(M,\varphi)$ of constant functor $M:\mathcal{I}\to \mathcal{C}$ and natural transformation $\varphi: G\to M$ , there is a unique natural transformation $\phi: R\to M$ such that the following diagram

The colimit is also known as the direct limit, right limit, inductive limit, coroot, or right root, and is written $\limdir$ .

Examples. Some common examples of limits (inverse limits) are products, terminal objects, pullbacks, equalizers, kernels, and kernel pairs. These examples can be readily verified. Let us verify that a terminal object is a limit:

Let $G$ be the empty functor (from the empty category) into an arbitrary category $\mathcal{C}$ . So the limit of $G$ is just an object $C$ in $\mathcal{C}$ , and that's it, as there are no objects in the empty category, there are no morphisms from $C$ in the limit of $G$ . If $A$ is any object in $\mathcal{C}$ , then there is a unique morphism $A\to C$ , and that's it. But this means that $C$ is just a terminal object of $\mathcal{C}$ .

Please see the verification of some of these examples in the attachments below.

Some examples of colimits (direct limits) are coproducts, initial objects, pushouts, coequalizers, cokernels, and cokernel pairs.