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I'm asked to prove that every totally disconnected topological space satisfies $T_1$ seperation axiom. my proof is this: fix $x\in X$. now, for each $y\in X\setminus\{x\} $ we know that $\left\{ x,y\right\}$ is not connected, therefore there are non-empty disjoint open sets in $X$, say $U,V_{y}$ such that $\left(U\cap\{ x,y\} \right)\cup\left(V_y \cap \{x,y\} \right)=\{x,y\} $. WLOG we can assume that $x\in U, y\in V_{y}$ and we get

$$X\setminus\{x\} =\bigcup_{y\in X\setminus\{x\} } V_y$$

hence every singleton is closed and $X$ is $T_1$

I've seen different proofs and I would like to know if this one is correct.

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  • $\begingroup$ When you use \backslash instead of \setminus, you don't get the horizontal spacing that is proper for a binary operation symbol. I edited accordingly. Thus I changed $X\backslash\{x\}$ to $X\setminus\{x\}.$ (I actually prefer $X\smallsetminus\{x\},$ but that's another matter.) $\endgroup$ Commented Aug 8, 2021 at 21:23
  • $\begingroup$ didn't know it, thank you! $\endgroup$
    – Oria
    Commented Aug 8, 2021 at 21:39

1 Answer 1

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$U$ and $V_y$ are not necessarily disjoint in $X$. You only have $U \cap \{x,y \} = \{x \}$ and $V \cap \{x,y \} = \{y \}$.

The rest of the proof is fine.

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  • $\begingroup$ Yes are right, thank you. however they are disjoint in $\left\{ x,y\right\} $ so the argument should still hold $\endgroup$
    – Oria
    Commented Aug 8, 2021 at 20:11
  • $\begingroup$ Yes, indeed, the argument still holds. $\endgroup$ Commented Aug 8, 2021 at 20:12
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    $\begingroup$ Note if they were disjoint @cos_dm_math21 you would have gotten T2, not T1, and totally disconnected does not guarantee T2. math.stackexchange.com/questions/1240439/… $\endgroup$
    – Alan
    Commented Aug 8, 2021 at 21:26
  • $\begingroup$ @Alan great insight! $\endgroup$
    – Oria
    Commented Aug 8, 2021 at 21:40

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