Friday 8 June 2007

gn.general topology - A question about the dispersion points of connected metric spaces

Here I construct an example which proves the answer is NO.



Take the KK fan:



enter image description here



Remove its dispersion point at the top. Now you have a Cantor set of lines of rationals/irrationals that cannot be separated horizontally. Stretch this into a "Cantor-like tube" and weave it closer and closer to a point $p$ in the plane while shrinking its diameter and making sure that every loop goes a distance of $7$ away from $p$.



enter image description here



Remove $p$ and you have a hereditarily disconnected space ($simeq$ KK fan minus its vertex). If $A$ is nonempty and clopen in $X$ then $A$ must snake around the tube forever, so it limits to $p$ and thus $pin A$. Therefore $X$ is connected.
The ball of radius $1$ around $p$ is hereditarily disconnected.



EDIT: I am basically taking the space which consists of the curve below, and the origin $p=(0,0)$ (so ${0}times (0,1]$ is not included). The difference is that instead of weaving a line, I am weaving this "Cantor tube" while shrinking its diameter. In the first case if I remove $p$ then I get something $simeq [0,infty)$, whereas in the second case I get something $simeq$ my Cantor tube.



enter image description here

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