[Math] weakly locally connected

connectednessgeneral-topology

A space $X$ is said to be weakly locally connected at $x$ if for every neighborhood of $x$, there is a connected subspace of $X$ contained in $U$ that contains a neighborhood of $x$. Show that if $X$ is weakly locally connected at each of its points, then $X$ is locally connected.(hint: Show that components of open sets are open.)

Proving the hint:

Let, $C$ be a component of a open set of $X$. Let $x\in C$. Now, let $U$ be any neighborhood of $x$, then being weakly locally connected, you can find another nbh. of $x$ which contains inside a connected subspace, hence that nbh is also connected open in $X$. This is true for each $x$. Hence $C$ must be open set.

But, how this proves the main result? Thanks.

Best Answer

The fact that $X$ is locally connected iff for all open subsets $O$ of $X$, all components of $O$ (as a subspace) are open in $O$ (and $X$ too), should be known to you. It's Munkres theorem 25.3

So we will show $X$ to be locally connected using the right to left direction of this theorem.: let $O$ be open in $X$ and let $C$ be a component of $O$ (in the subspace topology).

Let $x \in C$. Then $x \in O$ and $O$ is a neighbourhood of $x$ so by assumption of weak locally connectedness there is a connected set $C_x \subseteq O$ such that $x \in \operatorname{int}(C_x)$ (as $C_x$ is a a neighbourhood of $x$).

Then $C$ and $C_x$ are both connected subsets of $O$ that intersect (in $x$) so $C \cup C_x$ is also a connected subset of $O$. As $C$ is a component of $O$ and as such is a maximally connected subset of $O$,

$$C \cup C_x = C \text{ so } x \in \operatorname{int}(C_x) \subseteq C$$

showing finally that $x$ is an interior point of $C$. As this holds for all $x \in C$, $C$ is open, as required.

Related Question