Timeline for Topological spaces without retracts [closed]

8 events

when toggle format	what		by	license	comment
Nov 23, 2018 at 21:51	history	closed	abx Igor Belegradek YCor Noah Schweber Yemon Choi		Needs more focus
Nov 23, 2018 at 17:08	comment	added	YCor		$\mapsto$ means assignment, the functional notation is $\to$
Nov 23, 2018 at 15:49	comment	added	Najib Idrissi		I will assume that you mean: given $B \subset \Omega$, how to see if there exists a retraction $\Omega \to B$? (There is a reason quantifiers are usually put at the beginning...) Anyway, this is too broad. You have some tools from algebraic topology but otherwise, I doubt you can get a better answer than "it exists when it exists".
Nov 23, 2018 at 15:45	review	Close votes
Nov 23, 2018 at 21:51
Nov 23, 2018 at 15:41	history	edited	THC	CC BY-SA 4.0	added 19 characters in body
Nov 23, 2018 at 15:28	comment	added	Najib Idrissi		Can you put quantifiers in your question? Are you asking for spaces such that there exists a $B$ with a retraction $r : \Omega \to B$? (That's all of them.) Are you asking for spaces such that for all $B \subset \Omega$, there exists a retraction $\Omega \to B$? (Very few spaces.) Does subspace mean proper subspace, or any subspace? In any case I'm not sure that your question is research-level.
Nov 23, 2018 at 15:10	answer	added	Dominic van der Zypen		timeline score: 2
Nov 23, 2018 at 15:01	history	asked	THC	CC BY-SA 4.0