Path (topology)



In mathematics, a path in a topological space X is a continuous map f from the unit interval I = [0,1] to X
 * f : I &rarr; X.

The initial point of the path is f(0) and the terminal point is f(1). One often speaks of a "path from x to y" where x and y are the initial and terminal points of the path. Note that a path is not just a subset of X which "looks like" a curve, it also includes a parametrization. For example, the maps f(x) = x and g(x) = x2 represent two different paths from 0 to 1 on the real line.

A loop in a space X based at x &isin; X is a path from x to x. A loop may be equally well regarded as a map f : I &rarr; X with f(0) = f(1) or as a continuous map from the unit circle S1 to X
 * f : S1 &rarr; X.

This is because S1 may be regarded as a quotient of I under the identification 0 &sim; 1. The set of all loops in X forms a space called the loop space of X.

A topological space for which there exists a path connecting any two points is said to be path-connected. Any space may be broken up into a set of path-connected components. The set of path-connected components of a space X is often denoted &pi;0(X);.

One can also define paths and loops in pointed spaces, which are important in homotopy theory. If X is a topological space with basepoint x0, then a path in X is one whose initial point in x0. Likewise, a loop in X is one that is based at x0.

Homotopy of paths
Paths and loops are central subjects of study in the branch of algebraic topology called homotopy theory. A homotopy of paths makes precise the notion of continuously deforming a path while keeping its endpoints fixed.

Specifically, a homotopy of paths, or path-homotopy, in X is a family of paths ft : I &rarr; X indexed by I such that The paths f0 and f1 connected by a homotopy are said to homotopic (or more precisely path-homotopic, to distinguish between the relation defined on all continuous functions between fixed spaces). One can likewise define a homotopy of loops keeping the base point fixed.
 * ft(0) = x0 and ft(1) = x1 are fixed.
 * the map F : I &times; I &rarr; X given by F(s, t) = ft(s) is continuous.

The relation of being homotopic is an equivalence relation on paths in a topological space. The equivalence class of a path f under this relation is called the homotopy class of f, often denoted [f].

Path composition
One can compose paths in a topological space in an obvious manner. Suppose f is a path from x to y and g is a path from y to z. The path fg is defined as the path obtained by first traversing f and then traversing g:
 * $$fg(s) = \begin{cases}f(2s) & 0\leq s \leq \frac{1}{2} \\ g(2s-1) & \frac{1}{2} \leq s \leq 1.\end{cases}$$

Clearly path composition is only defined when the terminal point of f coincides with the initial point of g. If one considers all loops based at a point x0, then path composition is a binary operation.

Path composition, whenever defined, is not associative due to the difference in parametrization. However it is associative up to path-homotopy. That is, [(fg)h] = [f(gh)]. Path composition defines a group structure on the set of homotopy classes of loops based at a point x0 in X. The resultant group is called the fundamental group of X based at x0, usually denoted &pi;1(X,x0).

In situations calling for associativity of path composition "on the nose," a path in X may instead be defined as a continuous map from an interval [0,a] to X for any real a &ge; 0. A path f of this kind has a length |f| defined as a. Path composition is then defined as before with the following modification:
 * $$fg(s) = \begin{cases}f(s) & 0\leq s \leq |f| \\ g(s-|f|) & |f| \leq s \leq |f|+|g|\end{cases}$$

Whereas with the previous definition, f, g, and fg all have length 1 (the length of the domain of the map), this definition makes |fg| = |f| + |g|. What made associativity fail for the previous definition is that although (fg)h and f(gh) have the same length, namely 1, the midpoint of (fg)h occurred between g and h, whereas the midpoint of f(gh) occurred between f and g. With this modified definition (fg)h and f(gh) have the same length, namely |f|+|g|+|h|, and the same midpoint, found at (|f|+|g|+|h|)/2 in both (fg)h and f(gh); more generally they have the same parametrization throughout.

Fundamental groupoid
There is a categorical picture of paths which is sometimes useful. Any topological space X gives raise to a category where the objects are the points of X and the morphisms are the homotopy classes of paths. Since any morphism in this category is an isomorphism this category is a groupoid, called the fundamental groupoid of X. Loops in this category are the endomorphisms (all of which are actually automorphisms). The automorphism group of a point x0 in X is just the fundamental group based at X. More generally, one can define the fundamental groupoid on any subset A of X, using homotopy classes of paths joining points of A. This is convenient for the Van_Kampen's_Theorem.

Reference

 * Ronald Brown, Topology and groupoids, Booksurge PLC, (2006).