Path space fibration explained

(X, *) ^[1] is a fibration of the form\Omega X \hookrightarrow PX \overset{\chi \mapsto \chi( 1 )}\to X wherePX is the based path space of the pointed space (X, *) ; that is, PX = \{ f\colon I \to X \mid f continuous , f( 0 ) = * \} equipped with the compact-open topology.\Omega X is the fiber of \chi \mapsto \chi( 1 ) over the base point of (X, *) ; thus it is the loop space of (X, *) .

The free path space of X, that is,

\operatorname{Map}(I, X) = X ^{I} , consists of all maps from I to X that do not necessarily begin at a base point, and the fibration X ^{I} \to X given by, say, \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 fiber of the based fibration is called the mapping fiber or, equivalently, the homotopy fiber.

Mapping path space

If

f\colon X\to Y is any map, then the mapping path space P _{f} of f is the pullback of the fibration Y ^{I} \to Y, \chi \mapsto \chi( 1 ) along f . (A mapping path space satisfies the universal property that is dual to that of a mapping cylinder, which is a push-out. Because of this, a mapping path space is also called a mapping cocylinder.)

Since a fibration pulls back to a fibration, if Y is based, one has the fibration

F _{f} \hookrightarrow P _{f} \overset{p}\to Y where p(x, \chi) = \chi( 0 ) and F _{f} is the homotopy fiber, the pullback of the fibration PY \overset{\chi \mapsto \chi( 1 )}{\longrightarrow} Y along f .

Note also

f is the compositionX \overset{\phi}\to P _{f} \overset{p}\to Y where the first map \phi sends x to (x, c _{f(x)} ) ; here c _{f(x)} denotes the constant path with value f(x) . Clearly, \phi is a homotopy equivalence; thus, the above decomposition says that any map is a fibration up to homotopy equivalence.

If

f is a fibration to begin with, then the map \phi\colon 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 f .

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

\alpha, \beta such that \alpha( 1 ) = \beta( 0 ) is the path \beta ⋅ \alpha\colon I \to X given by:(\beta ⋅ \alpha)(t) = \begin{cases} \alpha( 2 t) & if 0 \le t \le 1 / 2 \\ \beta( 2 t -1 ) & if 1 / 2 \le t \le 1 \\ \end{cases} .This product, in general, fails to be associative on the nose: (\gamma ⋅ \beta) ⋅ \alpha \ne \gamma ⋅ (\beta ⋅ \alpha) , as seen directly. One solution to this failure is to pass to homotopy classes: one has [(\gamma ⋅ \beta) ⋅ \alpha] = [\gamma ⋅ (\beta ⋅ \alpha)] . Another solution is to work with paths of arbitrary lengths, leading to the notions of Moore's path space and Moore's path space fibration, described below. (A more sophisticated solution is to rethink composition: work with an arbitrary family of compositions; see the introduction of Lurie's paper,^[3] leading to the notion of an operad.)

Given a based space

(X, *) , we letP' X = \{ f\colon [ 0 , r] \to X \mid r \ge 0 , f( 0 ) = * \}. An element f of this set has a unique extension \widetilde{f} to the interval [ 0 , inf ty) such that \widetilde{f}(t) = f(r), t \ge r . Thus, the set can be identified as a subspace of \operatorname{Map}([ 0 , inf ty), X) . The resulting space is called the Moore path space of X, after John Coleman Moore, who introduced the concept. Then, just as before, there is a fibration, Moore's path space fibration:\Omega' X \hookrightarrow P'X \overset{p}\to X where p sends each f: [ 0 , r] \to X to f(r) and \Omega' X = p ^{-1} (*) is the fiber. It turns out that \Omega X and \Omega' X are homotopy equivalent.

Now, we define the product map

\mu: P' X x \Omega' X \to P' X by: for f\colon [ 0 , r] \to X and g\colon [ 0 , s] \to X ,\mu(g, f)(t) = \begin{cases} f(t) & if 0 \le t \le r \\ g(t - r) & if r \le t \le s + r \\ \end{cases} .This product is manifestly associative. In particular, with μ restricted to ΩX × ΩX, we have that ΩX is a topological monoid (in the category of all spaces). Moreover, this monoid ΩX acts on PX through the original μ. In fact, p: P'X \to X is an Ω'X-fibration]].[4]

References

• Book: James F. . Davis. Paul. Kirk. Lecture Notes in Algebraic Topology. Graduate Studies in Mathematics. 35. American Mathematical Society. Providence, RI. 2001. xvi+367. 0-8218-2160-1 . 1841974. 10.1090/gsm/035.
• Book: May, J. Peter. J. Peter May

. J. Peter May. A Concise Course in Algebraic Topology. Chicago Lectures in Mathematics. University of Chicago Press. Chicago, IL. 1999. x+243. 0-226-51182-0. 1702278.

• Book: Whitehead, George W.. George W. Whitehead

. George W. Whitehead. Elements of homotopy theory. 3rd. Graduate Texts in Mathematics. 61. 1978. Springer-Verlag. New York-Berlin. 978-0-387-90336-1. xxi+744. 0516508 .

Notes and References

1. Throughout the article, spaces are objects of the category of "reasonable" spaces; e.g., the category of compactly generated weak Hausdorff spaces.
2. using the change of fiber
3. Web site: Jacob. Lurie. Jacob Lurie. Derived Algebraic Geometry VI: E[k]-Algebras]. October 30, 2009.
4. Let G = ΩX and P = PX. That G preserves the fibers is clear. To see, for each γ in P, the map G \to p ^{-1} (p(\gamma)), g \mapsto \gamma g is a weak equivalence, we can use the following lemma:We apply the lemma with B = I, D = I x G, E = I x _{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 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.)