# Commit 2024-09-29 14:59 e060e71a

View on Github →feat(Topology): define (relative) CW-complexes (#12502)
We define a relative CW-complex as the colimit of an expanding sequence of subspaces `sk i`

(called the $(i-1)$-skeleton) for `i ≥ 0`

, where `sk 0`

(i.e., the $(-1)$-skeleton) is an arbitrary topological space, and each `sk (n + 1)`

(i.e., the $n$-skeleton) is obtained from `sk n`

(i.e., the $(n-1)$-skeleton) by attaching `n`

-disks.
This is based on David Wärn's suggestion on Zulip and Chapter 10 of *A Concise Course in Algebraic Topology* by J. P. May.