# The Unapologetic Mathematician

## Cones and cocones

There are a few auxiliary concepts we’ll need before the next major topic. Let’s start with two categories $\mathcal{J}$ and $\mathcal{C}$, and the category of functors $\mathcal{C}^\mathcal{J}$. If the following seems very complicated, consider $\mathcal{J}$ to be any particular toy category you’d like, so this category of functors is a diagram category.

Now, for every object $C\in\mathcal{C}$ there is a constant functor that sends every object of $\mathcal{J}$ to $C$, and every morphism to the identity on $C$. Actually, this assignment of the constant functor to an object of $C$ is a functor from $\mathcal{C}$ to $\mathcal{C}^\mathcal{J}$. Indeed, given a morphism $f:C\rightarrow D$ we get a natural transformation from the one constant functor to the other, whose component at each object of $\mathcal{J}$ is $f$. We call this the “diagonal” functor $\Delta:\mathcal{C}\rightarrow\mathcal{C}^\mathcal{J}$. That is, $\Delta(C)$ is the constant functor with value $C$.

Let $F:\mathcal{J}\rightarrow\mathcal{C}$ be some particular functor in $\mathcal{C}^\mathcal{J}$. A cone is an object in the comma category $(\Delta\downarrow F)$. Let’s unpack this definition. Since $\Delta$ has $\mathcal{C}$ as its source, and $F$ is a fixed object — the same thing as a functor from the category $\mathbf{1}$ to $\mathcal{C}^\mathcal{J}$, we know what objects of this category look like.

An object of the category $(\Delta\downarrow F)$ consists of an object $C$ of $\mathcal{C}$ and an arrow from $\Delta(C)$ to $F$. But an arrow in $\mathcal{C}^\mathcal{J}$ is a natural transformation of functors. That is, for each object $J$ of $\mathcal{J}$ we need an arrow in $\mathcal{C}$ from $\left[\Delta(C)\right](J)$ to $F(J)$. But $\left[\Delta(C)\right](J)=C$ for all objects $J$. So we just need an object $C\in\mathcal{C}$ and an arrow $C\rightarrow F(J)$ for each $J\in\mathcal{J}$.

Of course, there’s also naturality conditions to be concerned with. If $j:J_1\rightarrow J_2$ is an arrow in $\mathcal{J}$, and $c_1:C\rightarrow F(J_1)$ and $c_2:C\rightarrow F(J_2)$ are the required arrows from $C$, then naturality requires that $c_2=F(j)\circ c_1$. So we need a collection of arrows from $C$ to the objects in the image of $F$ that are compatible with the arrows from $\mathcal{J}$. Such a collection defines an object in the comma category $(\Delta\downarrow F)$ — a cone on $F$.

Cocones are defined similarly. A cocone on $F$ is an object in the comma category $(F\downarrow\Delta)$. That is, it’s an object $C\in\mathcal{C}$ and a collection of arrows from the objects in the image of $F$ that are compatible with the arrows from $\mathcal{J}$.

The description may seem a little odd, but try writing it out for some very simple categories $\mathcal{J}$. For example, let $\mathcal{J}$ be a set. Then try letting it be an ordinal, or another preorder. After you write down the definition of a cone and a cocone on some simple categories the general idea should seem to make more sense.

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Posted by John Armstrong | Category theory

## 4 Comments	»

1. [...] a functor the limit of (if it exists) is a couniversal cone on — a terminal object in the comma category . That is, it consists of an object and arrows [...]

Pingback by | June 19, 2007 | Reply

2. [...] are Adjoints When considering limits, we started by talking about the diagonal functor . This assigns to an object the “constant” functor that sends each object of to and [...]

Pingback by | July 20, 2007 | Reply

3. 2 ‘formula does not parse’ latex errors here

Comment by Avery | January 5, 2008 | Reply

4. Yeah, those have been creeping in. WordPress changed something in their parser and now brackets give trouble unless they’re given as \left[ \right] pairs. I don’t know which pages have the problems, but when I stumble across them I fix them. Thanks for pointing this example out to me.

Comment by | January 6, 2008 | Reply

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