filtered topological space



A filtered topological space X *X_* is a filtered object in Top, hence

  1. a topological space X=X X=X_\infty

  2. equipped with a sequence of subspaces

    X *:=X 0X 1X nX .X_*:= \quad X_0 \subseteq X_1 \subseteq \cdots \subseteq X_n \subseteq \cdots \subseteq X_\infty.

A filtered space X *X_* is called a connected filtered space if it satisfies the following condition:

(ϕ) 0(\phi)_0: The function π 0X 0π 0X r\pi_0X_0 \to \pi_0 X_r induced by inclusion is surjective for all r0r \geq 0; and, for all i1i \geq 1, (ϕ i):π i(X r,X i,v)=0(\phi_i): \pi_i(X_r,X_i,v)=0 for all r>ir \gt i and vX 0 v \in X_0.

There are two other forms of this condition which are useful under different circumstances.


  1. A CW-complex XX with its filtration by skeleta X nX^n.

  2. The free topological monoid FXF X on a space XX filtered by the length of words. Given a based space (X,x)(X,x), there is also a reduced version by taking FXF X and identifying xx with the identity of FXF X. This latter filtered space is known as the James construction J(X,x)J(X,x), after Ioan James.

  3. A similar example to the last using free groups instead of free monoids.

  4. A similar example to the last using free groupoids on topological graphs.

  5. A similar example to the last using the universal topological groupoid U σ(G)U_\sigma(G) induced from a topological groupoid GG by a continuous function f:Ob(G)Yf: Ob(G) \to Y to a space YY.

Examples of connected filtered spaces are:

  1. The skeletal filtration of a CW-complex.

  2. The word length filtration of the James construction for a space with base point such that {x}X\{x\} \to X is a closed cofibration.

  3. The filtration (BC) *(B C)_* of the classifying space of a crossed complex, filtered using skeleta of CC.

This condition occurs in the higher homotopy van Kampen theorem for crossed complexes.


We thus see that filtered spaces arise from many geometric and algebraic situations, and see also stratified spaces). It is therefore interesting that one can define strict higher homotopy groupoids for filtered spaces more easily than for spaces themselves.

Note also that it is standard to be able to replace, using mapping cylinders, a sequence of maps Y nY n+1Y_n \to Y_{n+1} by a sequence of inclusions.

Revised on August 23, 2013 23:00:08 by Tim Porter (