Wednesday, 7 November 2018

general topology - "Basis" Version of a Local Connectedness Theorem





A space X is locally connected if and only if for every open set U of X, each component of U is open in X.




The above is a theorem in Munkres' Topology book (theorem 25.3). As usual, I always wonder whether a given theorem is true when open sets are replaced with basis elements. I just proved that X is locally connected if and only if X has a basis entirely comprised of connected sets, thinking that this might help. But I cannot quite connect the pieces. So my question is




Does a "basis" version of the above theorem hold?



Answer




I don't imagine the proof to be that different from the original version: Let X be a topological space with basis B. Suppose for every BB we have that each component of B is open in X. Now take any xX and any open neighbourhood U of x. By definition there is a BB such that xBU. Now let C be the component of B containing x. Then xCU and C is open and connected. So X is locally connected.



To prove the converse assume X is locally connected. By the above theorem, we have that every open UX, each component of U is open. Since every BB is open, each component of BB must be open too.


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