Showing that there is no immediate deformation retraction to a point – math.stackexchange.com
kaba
Let
$$X = \overline{B}_{n + 1},$$
$$C = \{x \in X: (\forall i \in (n + 1) \setminus \{0\}: x_i = 0) \land x_0 \geq 0\},$$
$$A = S_n \cup C,$$
where
$\overline{B}_{n + 1}$ is the closed unit $(n + ...