Path space fibration

In algebraic topology, the path space fibration over a based space (X, *)^[1] is a fibration of the form

${\displaystyle \Omega X\hookrightarrow PX{\overset {\chi \mapsto \chi (1)}{\to }}X}$

where

• ${\displaystyle PX=\operatorname {Map} (I,X)=\{f:I\to X|f(0)=*\}}$ is the space called the path space of X.
• $\Omega X$ is the fiber of ${\displaystyle \chi \mapsto \chi (1)}$ over the base point of X; thus it is the loop space of X.

The space ${\displaystyle X^{I}}$ consists of all maps from I to X that may not preserve the base points; it is called the free path space of X and the fibration ${\displaystyle X^{I}\to X}$ given by, say, ${\displaystyle \chi \mapsto \chi (1)}$, is called the free path space fibration.

The path space fibration can be understood to be dual to the mapping cone. The reduced fibration is called the mapping fiber or, equivalently, the homotopy fiber.

Mapping path space

If ƒ:X→Y is any map, then the mapping path space P_ƒ of ƒ is the pullback of ${\displaystyle Y^{I}\to Y,\,\chi \mapsto \chi (1)}$ along ƒ. Since a fibration pullbacks to a fibration, if Y is based, one has the fibration

${\displaystyle F_{f}\hookrightarrow P_{f}{\overset {p}{\to }}Y}$

where ${\displaystyle p(x,\chi )=\chi (0)}$ and $F_f$ is the homotopy fiber, the pullback of ${\displaystyle PY{\overset {\chi \mapsto \chi (1)}{\to }}Y}$ along ƒ.

Note also ƒ is the composition

${\displaystyle X{\overset {\phi }{\to }}P_{f}{\overset {p}{\to }}Y}$

where the first map φ sends x to ${\displaystyle (x,c_{f(x)})}$, ${\displaystyle c_{f(x)}}$ the constant path with value ƒ(x). Clearly, φ is a homotopy equivalence; thus, the above decomposition says that any map is a fibration up to homotopy equivalence.

If ƒ is a fibration to begin with, then ${\displaystyle \phi :X\to P_{f}}$ is a fiber-homotopy equivalence and, consequently,^[2] the fibers of f over the path-component of the base point are homotopy equivalent to the homotopy fiber $F_f$ of ƒ.

Moore's path space

By definition, a path in a space X is a map from the unit interval I to X. Again by definition, the product of two paths α, β such that α(1) = β(0) is the path β · α: I → X given by:

${\displaystyle (\beta \cdot \alpha )(t)={\begin{cases}\alpha (2t)&{\text{if }}0\leq t\leq 1/2\\\beta (2t-1)&{\text{if }}1/2\leq t\leq 1\\\end{cases}}}$.

This product, in general, fails to be associative on the nose: (γ · β) · α ≠ γ · (β · α), as seen directly. One solution to this failure is to pass to homotopy classes: one has [(γ · β) · α ] = [γ · (β · α)]. Another solution is to work with paths of arbitrary length, leading to the notions of Moore's path space and Moore's path space fibration.^[3]

Given a based space (X, *), we let

${\displaystyle P'X=\{f:[0,r]\to X|r\geq 0,f(0)=*\}.}$

An element f of this set has the unique extension $\widetilde {f}$ to the interval $[0,\infty )$ such that ${\displaystyle {\widetilde {f}}(t)=f(r),\,t\geq r}$. Thus, the set can be identified as a subspace of ${\displaystyle \operatorname {Map} ([0,\infty ),X)}$. The resulting space is called Moore's path space of X. Then, just as before, there is a fibration, Moore's path space fibration:

${\displaystyle \Omega 'X\hookrightarrow P'X{\overset {p}{\to }}X}$

where p sends each f: [0, r] → X to f(r) and ${\displaystyle \Omega 'X=p^{-1}(*)}$ is the fiber. It turns out that $\Omega X$ and ${\displaystyle \Omega 'X}$ are homotopy equivalent.

Now, we define the product map:

${\displaystyle \mu :P'X\times \Omega 'X\to P'X}$

by: for ${\displaystyle f:[0,r]\to X}$ and ${\displaystyle g:[0,s]\to X}$,

${\displaystyle \mu (g,f)(t)={\begin{cases}f(t)&{\text{if }}0\leq t\leq r\\g(t-r)&{\text{if }}r\leq t\leq s+r\\\end{cases}}}$.

This product is manifestly associative. In particular, with μ restricted to Ω'X × Ω'X, we have that Ω'X is a topological monoid (in the category of all spaces). Moreover, this monoid Ω'X acts on P'X through the original μ. In fact, ${\displaystyle p:P'X\to X}$ is an Ω'X-fibration.^[4]

Notes

1. Throughout the article, spaces are objects of the category of "reasonable" spaces; e.g., the category of compactly generated weak Haudsorff spaces.
2. using the change of fiber
3. Whitehead 1979, Ch. III, § 2.
4. Let G = Ω'X and P = P'X. That G preserves the fibers is clear. To see, for each γ in P, the map ${\displaystyle G\to p^{-1}(p(\gamma )),\,g\mapsto \gamma g}$ is a weak equivalence, we can use the following lemma:
   Lemma — Let p: D → B, q: E → B be fibrations over an unbased space B, f: D → E a map over B. If B is path-connected, then the following are equivalent:

   • f is a weak equivalence.
   • ${\displaystyle f:p^{-1}(b)\to q^{-1}(b)}$ is a weak equivalence for some b in B.
   • ${\displaystyle f:p^{-1}(b)\to q^{-1}(b)}$ is a weak equivalence for every b in B.
   We apply the lemma with ${\displaystyle B=I,D=I\times G,E=I\times _{X}P,f(t,g)=(t,\alpha (t)g)}$ where α is a path in P and I → X is t → the end-point of α(t). Since ${\displaystyle p^{-1}(p(\gamma ))=G}$ if γ is the constant path, the claim follows from the lemma. (In a nutshell, the lemma follows from the long exact homotopy sequence and the five lemma.)

References

• James F. Davis, Paul Kirk, Lecture Notes in Algebraic Topology
• May, J. A Concise Course in Algebraic Topology
• George William Whitehead (1978). Elements of homotopy theory. Graduate Texts in Mathematics. 61 (3rd ed.). New York-Berlin: Springer-Verlag. pp. xxi+744. ISBN 978-0-387-90336-1. MR 0516508. Retrieved September 6, 2011.