# Morphism of Diagrams

Throughout this article $\mathcal C$ denotes a category and J is an index category.

Definition

Given diagrams $D, D' : J\to \mathcal C$, a morphism $D \to D'$ is a natural transformation $T : D\Rightarrow D'$.

Thus we have the category of all diagrams in $\mathcal C$ of type J, which we will denote by $\mathcal C^J$.

For example if we write D and D’ as tuples:

$\left((A_i)_{i\in J}, (\beta_{e}: A_i \to A_j)_{(e : i\to j)}\right), \quad \left((A'_i)_{i\in J}, (\beta'_{e}: A'_i \to A'_j)_{(e : i\to j)}\right)$

a morphism $D\to D'$ is a collection of morphisms $\gamma_i : A_i \to A_i'$ such that

$(e:i\to j) \implies \beta'_e \circ \gamma_i = \gamma_j \circ \beta_e : A_i \to A_j'.$

In diagram form, we have the following, where all “rectangles” commute.

Exercise A

Suppose any diagram of type J in $\mathcal C$ has a colimit. Prove that we get a functor $\mathrm{colim} : \mathcal C^J \longrightarrow \mathcal C$ which takes a diagram in $\mathcal C$ to its colimit. In other words show that a morphism of two diagrams of same type in $\mathcal C$ induces a morphism of their colimits.

# Category of Modules

Proposition 1.

Colimits always exist in the category of A-modules.

Proof

Suppose $\left((M_i)_{i\in J}, (\beta_{e}: M_i \to M_j)_{(e : i\to j)}\right)$ is a diagram of type J. Let $P = \oplus_{i\in J} M_i$ with canonical embeddings $\nu_i : M_i \to P$. Let $Q\subseteq P$ be the submodule generated by all elements of the form $\nu_i(m_i) - \nu_j(\beta_e(m_i))$ over all $m_i \in M_i$ and $e : i\to j$ in J. We claim that $P/Q$ satisfies our desired universal properties. Define

$\epsilon_i : M_i \longrightarrow P/Q, \qquad M_i \stackrel {\nu_i}\longrightarrow \oplus_{i\in J} M_i = P \stackrel \pi \longrightarrow P/Q.$

By definition $\epsilon_j \circ \beta_e = \pi\circ \nu_j \circ \beta_e = \pi\circ \nu_i = \epsilon_i$ for all $e:i \to j$.

Now suppose we have a module N with linear maps $(\alpha_i : M_i \to N)_{i\in J}$ such that for any $e:i\to j$ we have $\alpha_j \circ \beta_e = \alpha_i$. The collection of $\alpha_i$ induce, by definition of direct sum, a unique map $g : \oplus_{i\in J} M_i \to N$ such that $g\circ\nu_i = \alpha_i$ for each $i\in J$. Hence

$e:i\to j \implies g\circ (\nu_j \circ \beta_e - \nu_i) = g\circ \nu_j \circ \beta_e - g\circ \nu_i = \alpha_j \circ \beta_e - \alpha_i = 0$

so $g$ factors through $f:P/Q \to N$ such that $f\circ \pi = g$. Thus for each $i\in J$ we have $f\circ \epsilon_i = f\circ \pi\circ \nu_i = g\circ \nu_i = \alpha_i$. ♦

Exercise B

Prove that colimits exist in the categories $\mathbf{Set}$, $\mathbf{Top}$ and $\mathbf{Gp}$.

# Another Functoriality

Definition.

Suppose $F: J_0 \to J$ is a morphism of index categories. Composition then gives:

$(D : J \to \mathcal C) \mapsto (D\circ F : J_0 \to \mathcal C)$.

Thus a diagram of type J gives us a diagram of type $J_0$. If $J_0$ is a subcategory of J, this is just the restriction of D to $J_0$, denoted by $D|_{J_0}$.

In fact we get a functor $F: \mathcal C^{J} \to \mathcal C^{J_0}$. Indeed, a morphism between diagrams $D_1, D_2 : J \to\mathcal C$ is a natural transformation $T : D_1\Rightarrow D_2$. We let F take this T to

$F(T) = 1_F * T : (D_1 \circ F) \Rightarrow (D_2\circ F)$,

where * is a form of “horizontal composition” of natural transformation (see the optional exercise here).

Although the abstract definition looks harrowing, the underlying concept is quite easy when $J_0$ is a subcategory of J, so it helps to keep this special case in mind. We denote the diagrams $D_1, D_2$ by the following tuples

$((A_i)_{i\in J}, (\beta_e : A_i \to A_j)_{e:i\to j}), \quad ((A'_i)_{i\in J}, (\beta'_e : A'_i \to A'_j)_{e:i\to j})$

so that a morphism $D_1 \to D_2$ is of a collection of morphisms $\gamma_i : A_i \to A_i'$ in $\mathcal C$ such that for any $e:i\to j$ we have $\beta'_e\circ \gamma_i = \gamma_j \circ \beta_e$. Now the new diagrams $D_1|_{J_0}$ and $D_2|_{J_0}$ are the same tuples but with $i\in J_0$ and $e:i\to j$ running through morphisms in $J_0$. Hence, $F(T)$ is given by the same collection of $\gamma_i$, except now i runs through $i \in J_0$.

Proposition 2.

Let $F : J_0 \to J$ be a morphism of index categories. Then for any diagram in $\mathcal C$ of type J, denoted by the pair $(A_i)$ and $(\beta_e)_{e:i\to j}$, we have an induced

$f: \mathrm{colim}_{i_0 \in J_0} A_{F(i_0)} \longrightarrow \mathrm{colim}_{i\in J} A_i$

assuming both colimits exist.

Proof

By definition $A := \mathrm{colim}_{i\in J} A_i$ comes with a collection of morphisms $(\epsilon_i : A_i \to A)_{i\in J}$ such that $\epsilon_j \circ \beta_e = \epsilon_i$ for all $e:i\to j$ in J.

Similarly $A_0 := \mathrm{colim}_{i_0 \in J_0} A_{F(i_0)}$ comes with a collection of morphisms $(\epsilon_{i_0} : A_{F(i_0)} \to A_0)_{i_0 \in J_0}$ such that $\epsilon_{j_0} \circ \beta_{F(e_0)} = \epsilon_{i_0}$ for all $e_0 : i_0 \to j_0$ in $J_0$.

From restricting the first colimit, we get a collection $(\epsilon_{F(i_0)} : A_{F(i_0)} \to A)_{i_0 \in J_0}$ such that $\epsilon_{F(j_0)} \circ \beta_{F(e_0)} = \epsilon_{F(i_0)}$ for all $e_0 : i_0 \to j_0$ in $J_0$.

By universal property of the colimit $A_0$, this induces a unique morphism $f:A_0 \to A$ such that $f\circ \epsilon_{i_0} = \epsilon_{F(i_0)}$ . ♦

Example

By restricting the following diagram

we obtain a morphism $B \coprod B' \to B \coprod_A B'$, assuming both objects exist. More generally we have $\coprod_{i \in J} A_i \to \mathrm{colim}_{i\in J} A_i$.

In fact, the proof of proposition 1 gives us a clue on how to construct general colimits. First take the coproduct, which corresponds to colimit over a diagram of vertices. Next we “add the arrow relations” by taking the coequalizer for each arrow.

# Limits

Limits are the dual of colimits.

Definition.

Take a diagram in $\mathcal C$ of type J, written as

$((A_i)_{i\in I}, (\beta_e : A_i \to A_j)_{(e:i\to j)}$

The limit of the diagram comprises of the following data:

$(A, (\pi_i : A \to A_i)_{i\in J})$

where $A = \lim_{i \in J} A_i \in \mathcal C$ is an object, $\pi_i : A \to A_i$ is a morphism in $\mathcal C$ for each $i\in J$, such that for any arrow $e:i\to j$, we have $\beta_e\circ\pi_i = \pi_j$.

We require the following universal property. For any tuple

$(B, (\alpha_i : B \to A_i)_{i\in J})$

where $B\in \mathcal C$ is an object, $\alpha_i$ is a morphism for each $i\in J$, such that for any arrow $e:i\to j$, we have $\beta_e \circ \alpha_i = \alpha_j$, there is a unique morphism $f : B \to A$ such that

$\pi_i\circ f = \alpha_i \text{ for each } i\in J.$

As before, we have the following special cases.

### Example 1: Products

If J is obtained from an index set I, the limit is the product $\prod_{i\in I} A_i$.

### Example 2: Pullbacks (Fiber Products)

If J is the following, the resulting limit is the pullback.

### Example 3: Equalizers

Definition.

The equalizer of $\beta_1, \beta_2 : A\to B$ in a category $\mathcal C$ is the limit of the following diagram.

This is a pair $(C, \pi : C\to A)$ such that $\beta_1\circ \pi = \beta_2\circ \pi$ and, for any pair $(D, \alpha : D\to A)$ such that $\beta_1 \circ \alpha = \beta_2 \circ \alpha$, there is a unique $f:D\to C$ such that $\pi\circ f = \alpha$.

Exercise C

Prove that limits always exist in the category of A-modules.

# Initial and Terminal Objects

Definition.

An object $A\in \mathcal C$ is said to be initial (resp. terminal) if for any object $B\in \mathcal C$, there is a unique morphism $A\to B$ (resp. $B\to A$).

Note

• If A is initial or terminal, there is a unique morphism $A\to A$, i.e. the identity.
• A is initial if and only if it is a (colimit / limit) of the empty diagram. [ Exercise: pick the right option and write the dual statement. ]

Exercise D

Prove that if A and A’ are initial, there is a unique isomorphism $A\to A'$. Dually, the same holds for terminal objects. In summary, initial (resp. terminal) objects are unique up to unique isomorphism.

The phrase “unique up to unique isomorphism” has been used multiple times while looking at universal properties. This is not a coincidence, for initial and terminal objects can be used to describe universal properties of various constructions. Here is an example.

Lemma 1.

Let M, N be A-modules. Consider the category $\mathcal C(M, N)$, whose objects are pairs

$(P, B : M\times N \to P)$

where P is an A-module, B is an A-bilinear map. The morphisms

$(B : M\times N \to P) \longrightarrow (B' : M\times N \to P')$

are A-linear maps $f:P\to P'$ such that $f\circ B = B'$. Then $(M\otimes_A N, (m, n)\mapsto m\otimes n)$ is an (initial / terminal) object in $\mathcal C$. [ Exercise: pick the right option. ]

Proof

Follows directly from the definition. ♦

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