nLab connected filtered space

Definition

A filtered space $X_*$ is called a connected filtered space if it satisfies:

1. $(\phi)_0$: The function $\pi_0X_0 \to \pi_0X_r$ induced by inclusion is surjective for all $r \gt 0$; and,

2. for all $i \geq 1$, $(\phi_i): \pi_i(X_r,X_i,v)=0$ for all $r \gt i$ and $v \in X_0$.

Another equivalent form is:

1. $(\phi_0')$: The function $\pi_0X_s \to \pi_0X_r$ induced by inclusion is surjective for all $0=s \lt r$ and bijective for all $1 \leq s \leq r$; and,

2. for all $i \geq 1$, $(\phi_i'): \, \pi_j(X_r,X_i,v)=0$ for all $v \in X_0$ and all $j,r$ such that $1 \leq j \leq i \lt r$.

Examples

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 $\{x\} \to X$ is a closed cofibration.

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

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