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 B∈B we have that each component of B is open in X. Now take any x∈X and any open neighbourhood U of x. By definition there is a B∈B such that x∈B⊂U. Now let C be the component of B containing x. Then x∈C⊂U 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 U⊂X, each component of U is open. Since every B∈B is open, each component of B∈B must be open too.
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