nLab
path

Definitions

In topology, a (parametrised, oriented) path in a space $X$ is a map (a morphism in an appropriate category of spaces) to $X$ from the unit interval $𝕀=[0,1]$. A path from $a$ to $b$ is a path $f$ such that $f(0)=a$ and $f(1)=b$. An unparametrised path is an equivalence class of paths, such that $f$ and $g$ are equivalent if there is an increasing automorphism $\varphi$ of $𝕀$ such that $g=f\circ \varphi$. An unoriented path is an equivalence class of paths such that $f$ is equivalent to $(x\mapsto f(1-x))$. A Moore path has domain $[0,n]$ for some natural number (or, more commonly, any non-negative real number) $n$. All of these variations can be combined, of course. (For unoriented paths, one usually says ‘between $a$ and $b$’ instead of ‘from $a$ to $b$’. Also, a Moore path from $a$ to $b$ has $f(n)=b$ instead of $f(1)=b$. Finally, there is not much difference between unparametrised paths and unparametrised Moore paths, since we may interpret $(t\mapsto nt)$ as a reparametrisation $\varphi$.)

In graph theory, a path is a list of edges, each of which ends where the next begins. Actually, this is a special case of the above, if we use Moore paths and interpret $[0,n]$ as the linear graph with $n+1$ vertices and $n$ edges; in this way, the other variations become meaningful. (However, as the only directed graph automorphism of $[0,n]$ is the identity, parametrisation is trivial for directed graphs and equivalent to orientation for undirected graphs. Note that a non-Moore path is simply an edge, one of the fundamental ingredients of a graph.)

Concatenation

Given a Moore path $f$ from $a$ to $b$ and a Moore path $g$ from $b$ to $c$, the concatenation of $f$ and $g$ is a Moore path $f;g$ or $g\circ f$ from $a$ to $c$. If the domain of $f$ is $[0,m]$ and the domain of $g$ is $[0,n]$, then the domain of $f;g$ is $[0,m+n]$, and

$$(f;g)(x)≔\{\begin{array}{cc}f(x)& x\le m\\ g(m+x)& x\ge m.\end{array}$$(f ; g)(x) \coloneqq \left \{ \array { f(x) & \quad x \leq m \\ g(m+x) & \quad x \geq m .} \right .

In this way, we get a (strict) category whose objects are points in $X$ and whose morphisms are Moore paths in $X$, with concatenation as composition. This category is called the Moore path category.

Often we are more interested in a quotient category of the Moore path category. If we use unparametrised paths (in which case we may use paths with domain $𝕀$ if we wish), then we get the unparametrised path category. If $X$ is a smooth space, then we may additionally identify paths related through a thin homotopy to get the path groupoid. Finally, if $X$ is a continuous space and we identify paths related through any (endpoint-preserving) homotopy, then we get the fundamental groupoid of $X$.

In graph theory, the Moore path category is known as the free category on the graph.

Related entries

• loop,
• path space,
• fundamental groupoid.