Topological functor

Source and sink

A source ${\displaystyle (X,(Y_{i})_{i\in I},(f_{i}\colon X\to Y_{i})_{i\in I})}$ in a category ${\displaystyle {\mathcal {E}}}$ consists of the following data:^{[2]: 125, Definition 1.1(1)}

• an object ${\displaystyle X\in {\mathcal {E}}}$,
• a (possibly proper) class of objects ${\displaystyle (Y_{i})_{i\in I}\subseteq {\mathcal {E}}}$
• and a class of morphisms ${\displaystyle (f_{i}\colon X\to Y_{i})_{i\in I}}$.

Dually, a sink ${\displaystyle (X,(Y_{i})_{i\in I},(f_{i}\colon Y_{i}\to X)_{i\in I})}$ in ${\displaystyle {\mathcal {E}}}$ consists of

• an object ${\displaystyle X\in {\mathcal {E}}}$,
• a class of objects ${\displaystyle (Y_{i})_{i\in I}\subseteq {\mathcal {E}}}$
• and a class of morphisms ${\displaystyle (f_{i}\colon Y_{i}\to X)_{i\in I}}$.

In particular, a source ${\displaystyle (f_{i}\colon X\to Y_{i})_{i\in I}}$ is an object ${\displaystyle X}$ if ${\displaystyle I}$ is empty, a morphism ${\displaystyle X\to Y}$ if ${\displaystyle I}$ is a set of a single element. Similarly for a sink.

Initial source and final sink

Let ${\displaystyle (f_{i}\colon X\to Y_{i})_{i\in I}}$ be a source in a category ${\displaystyle {\mathcal {E}}}$ and let ${\displaystyle \Pi \colon {\mathcal {E}}\to {\mathcal {B}}}$ be a functor. The source ${\displaystyle (f_{i})_{i\in I}}$ is said to be a ${\displaystyle \Pi }$-initial source if it satisfies the following universal property.^{[2]: Definition 2.1(1)}

• For every object ${\displaystyle X'\in {\mathcal {E}}}$, a morphism ${\displaystyle {\hat {g}}\colon \Pi (X')\to \Pi (X)}$ and a family of morphisms ${\displaystyle (f'_{i}\colon X'\to Y_{i})_{i\in I}}$ such that ${\displaystyle \Pi (f_{i})\circ {\hat {g}}=\Pi (f'_{i})}$ for each ${\displaystyle i\in I}$, there exists a unique ${\displaystyle {\mathcal {E}}}$-morphism ${\displaystyle g\colon X'\to X}$ such that ${\displaystyle {\hat {g}}=\Pi (g)}$ and ${\displaystyle \forall i\in I\colon f_{i}\circ g=f'_{i}}$.

  ${\displaystyle {\begin{matrix}{\mathcal {E}}&\qquad {\overset {\Pi }{\to }}\qquad &{\mathcal {B}}\\\hline {\begin{matrix}X'\\{\scriptstyle \exists !g}\downarrow {\color {White}\scriptstyle \exists !g}&\searrow \!\!^{f'_{i}}\!\!\!\!\!\!\\X&{\underset {f_{i}}{\to }}&Y_{i}\end{matrix}}&\qquad {\overset {\Pi }{\mapsto }}\qquad &{\begin{matrix}\Pi X'\\{\scriptstyle {\hat {g}}}\downarrow {\color {White}\scriptstyle {\hat {g}}}&\searrow \!\!^{\Pi f'_{i}}\!\!\!\!\!\!\\\Pi X&{\underset {\Pi f_{i}}{\to }}&\Pi Y_{i}\end{matrix}}\end{matrix}}}$

Similarly one defines the dual notion of ${\displaystyle \Pi }$-final sink.

When ${\displaystyle I}$ is a set of a single element, the initial source is called a Cartesian morphism.

Lift

Let ${\displaystyle {\mathcal {E}}}$, ${\displaystyle {\mathcal {B}}}$ be two categories. Let ${\displaystyle \Pi \colon {\mathcal {E}}\to {\mathcal {B}}}$ be a functor. A source ${\displaystyle ({\hat {f}}_{i}\colon {\hat {X}}\to {\hat {Y}}_{i})_{i\in I}}$ in ${\displaystyle {\mathcal {B}}}$ is a ${\displaystyle \Pi }$-structured source if for each ${\displaystyle i}$ we have ${\displaystyle {\hat {Y}}_{i}=\Pi (Y_{i})}$ for some ${\displaystyle Y_{i}\in {\mathcal {E}}}$.^{[2]: 128, Definition 1.1(2)} One similarly defines a ${\displaystyle \Pi }$-structured sink.

A lift of a ${\displaystyle \Pi }$-structured source ${\displaystyle ({\hat {f}}_{i}\colon {\hat {X}}\to \Pi (Y_{i}))_{i\in I}}$ is a source ${\displaystyle (f_{i}\colon X\to Y_{i})_{i\in I}}$ in ${\displaystyle {\mathcal {E}}}$ such that ${\displaystyle \Pi (X)={\hat {X}}}$ and ${\displaystyle \Pi (f_{i})={\hat {f}}_{i}}$ for each ${\displaystyle i\in I}$

${\displaystyle {\begin{matrix}{\mathcal {E}}&\qquad {\overset {\Pi }{\to }}\qquad &{\mathcal {B}}\\\hline {\begin{matrix}\exists X\\{\scriptstyle \exists f_{i}}\downarrow {\color {White}\scriptstyle \exists f_{i}}\\Y_{i}\end{matrix}}&\qquad {\overset {\Pi }{\mapsto }}\qquad &{\begin{matrix}{\hat {X}}\\{\scriptstyle {\hat {f}}_{i}}\downarrow {\color {White}\scriptstyle {\hat {f}}_{i}}\\\Pi Y_{i}\end{matrix}}\end{matrix}}}$

A lift of a ${\displaystyle \Pi }$-structured sink is similarly defined. Since initial and final lifts are defined via universal properties, they are unique up to a unique isomorphism, if they exist.

If a ${\displaystyle \Pi }$-structured source ${\displaystyle ({\hat {X}}\to \Pi (Y_{i}))_{i\in I}}$ has an initial lift ${\displaystyle (X\to Y_{i})_{i\in I}}$, we say that ${\displaystyle X}$ is an initial ${\displaystyle {\mathcal {E}}}$-structure on ${\displaystyle {\hat {X}}}$ with respect to ${\displaystyle ({\hat {X}}\to \Pi (Y_{i}))_{i\in I}}$. Similarly for a final ${\displaystyle {\mathcal {E}}}$-structure with respect to a ${\displaystyle \Pi }$-structured sink.

Topological functor

Let ${\displaystyle \Pi \colon {\mathcal {E}}\to {\mathcal {B}}}$ be a functor. Then the following two conditions are equivalent.^{[2]: 128, Definition 2.1(3) [3]: 29–30, §2 [4]: 2, Example 2.1(25) : 4, Definition 2.12}

• Every ${\displaystyle \Pi }$-structured source has an initial lift. That is, an initial structure always exists.
• Every ${\displaystyle \Pi }$-structured sink has a final lift. That is, a final structure always exists.

A functor satisfying this condition is called a topological functor.

One can define topological functors in a different way, using the theory of enriched categories.^[1]

A concrete category ${\displaystyle ({\mathcal {E}},F)}$ is called a topological (concrete) category if the forgetful functor ${\displaystyle F\colon {\mathcal {E}}\to \operatorname {Set} }$ is topological. (A topological category can also mean an enriched category enriced over the category ${\displaystyle \operatorname {Top} }$ of topological spaces.) Some require a topological category to satisfy two additional conditions.

• Constant functions in ${\displaystyle \mathbf {Set} }$ lift to ${\displaystyle {\mathcal {E}}}$-morphisms.
• Fibers ${\displaystyle \Pi ^{-1}({\hat {X}})}$ (${\displaystyle {\hat {X}}\in \mathbf {Set} }$) are small (they are sets and not proper classes).