# Difference between revisions of "0708-1300/the unit sphere in a Hilbert space is contractible"

Let $H=\{(x_1,x_2,...)| \sum x_n^2<\infty\}$ and define $S^{\infty}=\{x\in H| ||x||=1\}$

Claim

$S^{\infty}$ is contractible

Proof

Suppose $x=(x_1,x_2,...)\in S^{\infty}$ then $\sum x_n^2=1$

Define $F_1:S^{\infty}\times I\rightarrow S^{\infty}$ by $F_1(x,t)=((1-t)x_1+t\sqrt{x_1^2+x_2^2},(1-t)x_2,x_3,x_4,...)/||((1-t)x_1+t\sqrt{x_1^2+x_2^2},(1-t)x_2,x_3,x_4,...)||$

$F_2:S^{\infty}\times I\rightarrow S^{\infty}$ by $F_2(x,t)=((1-t)x_1+t\sqrt{x_1^2+x_3^2},0,(1-t)x_3,x_4,x_5,...)/||((1-t)x_1+t\sqrt{x_1^2+x_3^2},0,(1-t)x_3,x_4,x_5,...)||$

$F_3:S^{\infty}\times I\rightarrow S^{\infty}$ by $F_3(x,t)=((1-t)x_1+t\sqrt{x_1^2+x_4^2},0,0,(1-t)x_4,x_5,x_6,...)/||((1-t)x_1+t\sqrt{x_1^2+x_4^2},0,0,(1-t)x_4,x_5,x_6,...)||$

and so on ...

applying the homotopy $F_1$ in the time interval $[0,1/2]$, $F_2$ in the interval $[1/2,3/4]$, $F_3$ in $[3/4,5/6]$ etc...

we get the desired contraction to the point $(1,0,0,...)$.