## § CW Complexes and HEP

If $X$ is a CW complex and $A$ is a closed subcomplex, then it has the HEP.
A closed subcomplex is a union of closed cells of $X$ such that $X$ is obtained
by adding more cells to $A$.
#### § Lemma

If $e$ is a disk, then there is a continuous map from $e \times [0, 1]$ to
$\partial e \times [0, 1] \cup (e \times \{ 0 \})$.
#### § Lemma

If $X$ is obtained from $A$ by attaching one $k$-cell, then $(X, A)$ has HEP.
Given a homotopy $h_t: A \times [0, 1] \rightarrow Y$ and a new homotopy
$F_0: X \rightarrow Y$ such that $F_0|A = h_t$, we want to complete $F$
such that $F_t|A = h_t$.
The only part I don't know where to define $F$ on is the new added $e$ portion.
So I need to construct $H$ on $e \times [0, 1]$. Use the previous map to get to
$e \times [0, 1] \cup (e \times \{0\})$. This is in the domain of $F_0$ or $h_t$,
and thus we are done.
#### § CW Complexes have HEP

Induction on lemma. base case is empty set.
#### § Connected 1D CW Complex

*Theorem:* any connected 1D CW complex is homotopic to wedge of circles.
- Find a contractible subcomplex $A$ of $X$ that passes through all $0$ cells.
- By HEP, $X \simeq X/A$. $X/A$ has only one zero-cell and other one cells.One cells are only attached to zero cells. Hence, is a wedge of circles.
- The idea to find a contractible subcomplex is to put a partial order on theset of
*all* contractible cell complexes by inclusion. - Pick a maximal element with respect to this partial order.
- Claim: maximal element must contain all zero cells. Suppose not. Then I can add the new zero cell into the maximal element (why does it remain contractible? Fishy!)