limit points and closure for connected sets
The below theorem shows that adding limit points to a connectedset preserves connectedness.
Theorem 1.
Suppose is a connected set in a topological space.If , then is connected.In particular, is connected.
Thus, one way to prove that a space is connected is to find a densesubspace in which is connected.
Two touching closed balls in shows that this theorem does not holdfor the interior. Along the same lines, taking the closure
does notpreserve separatedness.
Proof.
Let be the ambient topological space.By assumption, if are open and , then.To prove that is connected, let be open sets in such that and for acontradition, suppose that .Then there are open sets such that
It follows that and .Next, let be open sets in defined as
Now
and as , it follows that.Then, by the properties of the closure operator,
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