nLab diagram

Contents

Contents

Idea

Informally, a diagram in a category CC consists of some objects of CC connected by some morphisms of CC. Frequently when doing category theory, we “draw diagrams” such as

A f B h k C g D\array{A & \overset{f}{\to} & B\\ ^h\downarrow && \downarrow^k\\ C& \underset{g}{\to} & D}

by drawing some objects (or dots labeled by objects) connected by arrows labeled by morphisms.

This terminology is often used when speaking about limits and colimits; that is, we speak about “the limit or colimit of a diagram.”

There are two natural ways to give the notion of “diagram” a formal definition. One is to say that a diagram is a functor, usually one whose domain is a (very) small category. This level of generality is sometimes convenient.

On the other hand, a more direct representation of what we draw on the page, when we “draw a diagram,” only involves labeling the vertices and edges of a directed graph (or quiver) by objects and morphisms of the category. This sort of diagram can be identified with a functor whose domain is a free category, and this is the most common context when we talk about diagrams “commuting.”

Definitions

Diagrams shaped like categories

We discuss here diagrams of the “shape of a small category”, as well as the concept of cones/cocones over these and limiting/colimiting (co-)cones. There is a quick abstract functorial definition

and there is a more long-winded but more explicit definition in terms of components

Functorial definition

We state the concise functorial definition of diagrams of the shape of categories.

Definition

(functorial definition)

Let 𝒞\mathcal{C} be a category and let \mathcal{I} small category,

Then

  1. a diagram XX of shape \mathcal{I} in 𝒞\mathcal{C} is a functor of the form

    X:𝒞, X \;\colon\; \mathcal{I} \longrightarrow \mathcal{C} \,,
  2. the category of \mathcal{I}-shaped diagrams in 𝒞\mathcal{C} is the functor category Funct(,𝒞)Funct(\mathcal{I}, \mathcal{C});

  3. a diagram X:𝒞X \colon \mathcal{I} \to \mathcal{C} is constant if it is a constant functor

    const X˜:!*X˜𝒞 const_{\tilde X} \;\colon\; \mathcal{I} \overset{\exists!}{\longrightarrow} \ast \overset{\tilde X}{\longrightarrow} \mathcal{C}

    in which case it is given by the data of a single object X˜\tilde X;

  4. a cone CC over a diagram X:𝒞X \colon \mathcal{I} \to \mathcal{C} with tip an object X˜𝒞\tilde X \in \mathcal{C} is a natural transformation from the constant diagram const X˜:*𝒞const_{\tilde X} \colon \mathcal{I} \to \ast \to \mathcal{C} to XX:

    C:const X˜X C \;\colon\; const_{\tilde X} \Rightarrow X
  5. a cocone CC under a diagram X:𝒞X \colon \mathcal{I} \to \mathcal{C} is a natural transformation to a constant diagram const X˜:*𝒞const_{\tilde X} \colon \mathcal{I} \to \ast \to \mathcal{C} from XX:

    C:Xconst X˜ C \;\colon\; X \Rightarrow const_{\tilde X}
  6. the limiting cone (or limit, for short) over a diagram XX is, if it exists, the terminal object in the category of cones over XX, which means that it is a cone C limC_{lim} with tip denoted lim iX i\underset{\longleftarrow}{\lim}_i X_i such that for every other cone CC with tip X˜\tilde X there is a unique natural transformation ϕ:const X˜const lim iX i\phi \colon const_{\tilde X} \Rightarrow const_{\underset{\longleftarrow}{\lim}_i X_i} such that

    C=C limϕ C = C_{lim} \circ \phi
  7. the colimiting cone (or colimit, for short) under a diagram XX is, if it exists, the initial object in the category of cocones under XX, which means that it is a co-cone C limC_{lim} with tip denoted lim iX i\underset{\longrightarrow}{\lim}_i X_i such that for every other cocone CC with tip X˜\tilde X there is a unique natural transformation ϕ:const lim iX iconst X˜\phi \colon const_{\underset{\longrightarrow}{\lim}_i X_i} \Rightarrow const_{\tilde X} such that

    C=ϕC lim. C = \phi \circ C_{lim} \,.

Component definition

We state an explicit component-based definition of diagrams of the shape of categories.

Definition

(diagram in a category)

A diagram X X_\bullet in a category is

  1. a set {X i} iI\{ X_i \}_{i \in I} of objects in the category;

  2. for every pair (i,j)I×I(i,j) \in I \times I of labels of objects a set {X if αX j} αI i,j\{ X_i \overset{ f_\alpha }{\longrightarrow} X_j\}_{\alpha \in I_{i,j}} of morphisms between these objects;

  3. for every label iIi \in I a choice of element ϵ iI i,i\epsilon_i \in I_{i,i};

  4. for each triple i,j,kIi,j,k \in I a function

    comp i,j,k:I i,j×I j,kI i,k comp_{i,j,k} \;\colon\; I_{i,j} \times I_{j,k} \longrightarrow I_{i,k}

such that

  1. the pairing compcomp is associative and unital with the f ϵ if_{\epsilon_i}-s the neutral elements;

  2. for every iIi \in I then f ϵ i=id X if_{\epsilon_i} = id_{X_i} is the identity morphism on the ii-th obect;

  3. for every composable pair of morphisms

X if αX jf βX k X_i \overset{f_{\alpha} }{\longrightarrow} X_j \overset{ f_{\beta} }{\longrightarrow} X_k

then the composite of these two morphisms equals the morphism of the diagram that is labeled by the value of comp i,j,kcomp_{i,j,k} on their labels:

f βf α=f comp i,j,k(α,β). f_{\beta} \circ f_\alpha \,=\, f_{comp_{i,j,k}( \alpha, \beta )} \,.

The last condition we depict as follows:

X j f α f β X i comp i,j,k(α,β) X k. \array{ && X_j \\ & {}^{\mathllap{f_{\alpha}}}\nearrow && \searrow^{\mathrlap{f_{\beta}}} \\ X_i && \underset{ comp_{i,j,k}(\alpha,\beta) }{\longrightarrow} && X_k } \,.
Definition

(cone over a diagram)

Consider a diagram

X =({X if αX j} i,jI,αI i,j,comp) X_\bullet \,=\, \left( \left\{ X_i \overset{f_\alpha}{\longrightarrow} X_j \right\}_{i,j \in I, \alpha \in I_{i,j}} \,,\, \mathrm{comp} \right)

in some category (def. ). Then

  1. a cone over this diagram is

    1. an object X˜\tilde X in the category;

    2. for each iIi \in I a morphism X˜p iX i\tilde X \overset{p_i}{\longrightarrow} X_i in the category

    such that

    • for all (i,j)I×I(i,j) \in I \times I and all αI i,j\alpha \in I_{i,j} then the condition

      f αp i=p j f_{\alpha} \circ p_i = p_j

      holds, which we depict as follows:

      X˜ p i p j X i f α X j \array{ && \tilde X \\ & {}^{\mathllap{p_i}}\swarrow && \searrow^{\mathrlap{p'_j}} \\ X_i && \underset{f_\alpha}{\longrightarrow} && X_j }
  2. a co-cone over this diagram is

    1. an object X˜\tilde X in the category;

    2. for each iIi \in I a morphism q i:X iX˜q_i \colon X_i \longrightarrow \tilde X in the category

    such that

    • for all (i,j)I×I(i,j) \in I \times I and all αI i,j\alpha \in I_{i,j} then the condition

      q jf α=q i q_j \circ f_{\alpha} = q_i

      holds, which we depict as follows:

      X i f α X j q i q j X˜. \array{ X_i && \overset{f_\alpha}{\longrightarrow} && X_j \\ & {}_{\mathllap{q_i}}\searrow && \swarrow_{\mathrlap{q_j}} \\ && \tilde X } \,.
Definition

Consider a diagram

X =({X if αX j} i,jI,αI i,j,comp) X_\bullet \,=\, \left( \left\{ X_i \overset{f_\alpha}{\longrightarrow} X_j \right\}_{i,j \in I, \alpha \in I_{i,j}} \,,\, \mathrm{comp} \right)

in some category (def. ). Then

  1. its limiting cone (or just limit for short) is, if it exists, the cone

    { lim iX i p i p j X i f α X j} \left\{ \array{ && \underset{\longleftarrow}{\lim}_i X_i \\ & {}^{\mathllap{p_i}}\swarrow && \searrow^{\mathrlap{p_j}} \\ X_i && \underset{f_\alpha}{\longrightarrow} && X_j } \right\}

    over this diagram (def. ) which is universal or terminal among all possible cones, in that it has the property that for

    { X˜ p i p j X i f α X j} \left\{ \array{ && \tilde X \\ & {}^{\mathllap{p'_i}}\swarrow && \searrow^{\mathrlap{p'_j}} \\ X_i && \underset{f_\alpha}{\longrightarrow} && X_j } \right\}

    any other cone, then there is a unique morphism

    ϕ:X˜lim iX i \phi \;\colon\; \tilde X \overset{}{\longrightarrow} \underset{\longrightarrow}{\lim}_i X_i

    that factors the given cone through the limiting cone, in that for all iIi \in I then

    p i=p iϕ p'_i = p_i \circ \phi

    which we depict as follows:

    X˜ ϕ p i lim iX i p i X i \array{ \tilde X \\ {}^{\mathllap{\phi}}\downarrow & \searrow^{\mathrlap{p_i}} \\ \underset{\longrightarrow}{\lim}_i X_i &\underset{p_i}{\longrightarrow}& X_i }
  2. its colimiting cocone (or just colimit for short) is, if it exists, the cocone

    {X i f α X j q i q j lim iX i} \left\{ \array{ X_i && \underset{f_\alpha}{\longrightarrow} && X_j \\ & {}^{\mathllap{q_i}}\searrow && \swarrow^{\mathrlap{q_j}} \\ \\ && \underset{\longrightarrow}{\lim}_i X_i } \right\}

    under this diagram (def. ) which is universal or initial among all possible co-cones, in that it has the property that for

    {X i f α X j q i q j X˜} \left\{ \array{ X_i && \underset{f_\alpha}{\longrightarrow} && X_j \\ & {}^{\mathllap{q'_i}}\searrow && \swarrow_{\mathrlap{q'_j}} \\ && \tilde X } \right\}

    any other cocone, then there is a unique morphism

    ϕ:lim iX iX˜ \phi \;\colon\; \underset{\longrightarrow}{\lim}_i X_i \overset{}{\longrightarrow} \tilde X

    that factors the given co-cone through the co-limiting cocone, in that for all iIi \in I then

    q i=ϕq i q'_i = \phi \circ q_i

    which we depict as follows:

    X i q i lim iX i ϕ q i X˜ \array{ X_i &\overset{q_i}{\longrightarrow}& \underset{\longrightarrow}{\lim}_i X_i \\ {}^{\mathllap{\phi}}\downarrow & \swarrow^{\mathrlap{q'_i}} \\ \tilde X }

Diagrams shaped like directed graphs

Definition

(free diagram)

If JJ is a directed graph with free category F(J)F(J), then a diagram in CC of shape JJ is a functor D:F(J)CD\colon F(J) \to C, or equivalently a graph morphism D¯:JU(C)\bar{D}\colon J \to U(C).

Here F:QuivCatF\colon Quiv \to Cat denotes the free category on a quiver and U:CatQuivU\colon Cat \to Quiv the underlying quiver of a category, which form a pair of adjoint functors. These are the sorts of diagrams which we “draw on a page” — we draw a quiver, and then label its vertices with objects of CC and its edges with morphisms in CC, thereby forming a graph morphism JU(C)J\to U(C).

Remarks

  • For either sort of diagram, JJ may be called the shape, scheme, or index category or graph.

  • Note that given a diagram D:JCD:J\to C, the image of the shape JJ is not necessarily a subcategory of CC, even if JJ is itself taken to be a category. This is because the functor DD could identify objects of JJ, thereby producing new potential composites which do not exist in JJ. (Sometimes one talks about the “image” of a functor as a subcategory, but this really means the subcategory generated by the image in the literal objects-and-morphisms sense.)

  • CC must be a strict category to make sense of U(C)U(C); however, F(J)F(J) always makes sense.

Commutative diagrams

If JJ is a category, then a diagram JCJ\to C is commutative if it factors through a thin category. Equivalently, a diagram of shape JJ commutes iff any two morphisms in CC that are assigned to any pair of parallel morphisms in JJ (i.e., with same source and target in JJ) are equal.

If JJ is a quiver, as is more common when we speak about “commutative” diagrams, then a diagram of shape JJ commutes if the functor F(J)CF(J) \to C factors through a thin category. Equivalently, this means that given any two parallel paths of arbitrary finite length (including zero) in JJ, their images in CC have equal composites.

Examples

  • The shape of the empty diagram is the initial category with no object and no morphism.

    Every category CC admits a unique diagram whose shape is the empty (initial) category, which is called the empty diagram in CC.

  • The shape of the terminal diagram is the terminal category J={*}J = \{*\} consisting of a single object and a single morphism (the identity morphism on that object).

    Specifying a diagram in CC whose shape is {*}\{*\} is the same as specifying a single object of CC, the image of the unique object of 11. (See global element)

  • A diagram of the shape {ab}\{a \to b\} in CC is the choice of any one morphism D ab:X aX bD_{a b} : X_a \to X_b in CC.

    Notice that strictly speaking this counts as a commuting diagram , but is a degenerate case of a commuting diagram, since there is only a single morphism involved, which is necessarily equal to itself.

  • If JJ is the quiver with one object aa and one endo-edge aaa\to a, then a diagram of shape JJ in CC consists of a single endomorphism in CC. Since aaa\to a and the zero-length path are parallel in JJ, such a diagram only commutes if the endomorphism is an identity. Note, in particular, that a single endomorphism can be considered as a diagram with more than one shape (this one and the previous one), and that whether this diagram “commutes” depends on the chosen shape.

  • A diagram of shape the poset indicated by

    {a b b c} \left\{ \array{ a &\to& b \\ \downarrow && \downarrow \\ b' &\to& c } \right\}

    is a commuting square in CC: this is a choice of four (not necessarily distinct!) objects X a,X b,X b,X cX_a, X_b, X_{b'}, X_c in C, together with a choice of (not necessarily distinct) four morphisms D ab:X aX bD_{a b} : X_a \to X_b, D bc:X bX cD_{b c} : X_b \to X_c and D ab:X aX bD_{a b'} : X_a \to X_{b'}, D bc:X bX cD_{b' c} : X_{b'} \to X_c in CC, such that the composite morphism D bcD abD_{b c}\circ D_{a b} equals the composite D bcD abD_{b' c}\circ D_{a b'}.

    One typically “draws the diagram” as

    X a D ab X b D ab D bc X b D bc X c \array{ X_a &\stackrel{D_{a b}}{\to}& X_b \\ {}^{\mathllap{D_{a b'}}}\downarrow && \downarrow^{\mathrlap{D_{b c}}} \\ X_{b'} &\stackrel{D_{b' c}}{\to}& X_{c} }

    in CC and says that the diagram commutes if the above equality of composite morphisms holds.

    Notice that the original poset had, necessarily, a morphism aca \to c and could have equivalently been depicted as

    {a b b c} \left\{ \array{ a &\to& b \\ \downarrow &\searrow& \downarrow \\ b' &\to& c } \right\}

    in which case we could more explicitly draw its image in CC as

    X a D ab X b D ab =D bcD abD bcD ab D bc X b D bc X c \array{ X_a &\stackrel{D_{a b}}{\to}& X_b \\ {}^{\mathllap{D_{a b'}}}\downarrow &\searrow^{\stackrel{D_{b c}\circ D_{a b}}{= D_{b' c}\circ D_{a b'}}}& \downarrow^{\mathrlap{D_{b c}}} \\ X_{b'} &\stackrel{D_{b' c}}{\to}& X_{c} }
  • By contrast, a diagram whose shape is the quiver

    {a b b c} \left\{ \array{ a &\to& b \\ \downarrow && \downarrow \\ b' &\to& c } \right\}

    is a not-necessarily-commuting square. The free category on this quiver differs from the poset in the previous example by having two morphisms aca\to c, one given by the composite abca\to b\to c and the other by the composite abca \to b'\to c. But the poset in the previous category is the poset reflection of this F(J)F(J), so a diagram of this shape commutes, in the sense defined above, iff it is a commuting square in the usual sense.

  • A pair of objects is a diagram whose shape is a discrete category with two objects.

  • A pair of parallel morphisms is a diagram whose shape is a category J={ab}J = \{a \stackrel{\to}{\to} b\} with two objects and two morphisms from one to the other.

    Notice that if we required {ab}\{a \stackrel{\to}{\to} b\} to be a poset this would necessarily make these two morphisms equal, and hence reduce this example to the one where J={ab}J = \{a \to b\}. In other words, a diagram of this shape only commutes if the two morphisms are equal.

  • A span is a diagram whose shape is a category with just three objects and single morphisms from one of the objects to the other two;

    J={ a b c} J = \left\{ \array{ && a \\ & \swarrow && \searrow \\ b &&&& c } \right\}

    dually, a cospan is a diagram whose shape is opposite to the shape of a span.

    J={b c a} J = \left\{ \array{ b &&&& c \\ & \searrow && \swarrow \\ && a } \right\}
  • A transfinite composition diagram is one of the shape the poset indicated by

    J={a 0 a 1 b}, J = \left\{ \array{ a_0 &\to& a_1 &\to& \cdots \\ & \searrow & \downarrow & \swarrow & \cdots \\ && b } \right\} \,,

    where the indices may range over the natural numbers or even some more general ordinal number.

    This is a non-finite commuting diagram.

  • tower diagram

Last revised on May 7, 2023 at 14:10:14. See the history of this page for a list of all contributions to it.