A topological space $X,\tau$ is said to be normal space if for each pair of disjoint closed sets $A$ and $B$, there exists open sets $U$ and $V$ such that $A\subseteq U$,$B\subseteq V$ and $U\cap V=\emptyset$. Prove that every metrizable space is normal space.

If $X,\tau$ is a metrizable space then there exists $d:X\times X\to[0,+\infty]$ that defines the open sets in $\tau$.

Consider $\epsilon=\frac{d(a,b)}{2}\forall a\in A$ and $b\in B$.

Then $A\subset \bigcup_{a\in A} \mathscr{B}(a,\epsilon)$ since A is closed. The affirmation is proven in the following way: some $a\in Fr(A)$ then $B(a,\epsilon)\cap Ext(A)\neq\emptyset$

In the same way $B\subset \bigcup_{b\in B} \mathscr{B}(b,\epsilon)$

If $U=\bigcup_{a\in A} \mathscr{B}(a,\epsilon)\\V=\bigcup_{b\in B} \mathscr{B}(b,\epsilon)$,

then $U\cap V=\emptyset$.

Therefore $(X,d)$ is a normal space.


Is my proof right? If not. Why?

Thanks in advance!

  • $\begingroup$ Looks good to me. $\endgroup$ – Yanko Nov 10 '18 at 15:37

The proof is wrong, as "$\varepsilon = \frac{d(a,b)}{2}\forall a \in a, \forall b \in B$" is ill-defined (the infimum of those numbers can be $0$ for disjoint closed sets).

Another, better idea is to note that $f_A: x \to d(x,A)$ is a continuous function from $X$ to $\mathbb{R}$ (where $d(x,A) = \inf \{ d(x,a): a \in A\}$ and which obeys $d(x,A) = 0$ iff $x \in \overline{A}$.

Then for disjoint closed $A$ and $B$, the function $$f(x) = \frac{d(x,A)}{d(x,A) + d(x,B)}$$ is well-defined (as no point $x$ exists such that $d(x,A) + d(x,B) = 0$, so that this term is always $>0$) and also continuous. Moreover $f[A] = \{0\}$ and $f[B]= \{1\}$ so that $U=f^{-1}[(-\infty,\frac13)]$ and $V = f^{-1}[(\frac{2}{3}, +\infty)]$ are disjoint open subsets of $A$ resp. $B$.

And $$A=(f_A)^{-1}[\{0\}] = \bigcap_n f^{-1}[(-\infty,\frac1n)]$$ shows that all closed sets are $G_\delta$'s and $(X,d)$ is even perfectly normal.

  • $\begingroup$ I am sorry but I am not understanding your proof. $\endgroup$ – Pedro Gomes Nov 10 '18 at 16:12
  • $\begingroup$ Could you explain what is the problem of the infimum of metric in my proof being 0? $\endgroup$ – Pedro Gomes Nov 10 '18 at 16:13
  • $\begingroup$ @PedroGomes you cannot have a ball of radius $0$. $\endgroup$ – Henno Brandsma Nov 10 '18 at 16:14
  • $\begingroup$ @PedroGomes what don't you understand? There are lots of posts showing the continuity of $d(x,A)$ as a function of $x$. $\endgroup$ – Henno Brandsma Nov 10 '18 at 16:15
  • $\begingroup$ I have proven that continuity. But I do not understand why you define the metric $f(x) = \frac{d(x,A)}{d(x,A) + d(x,B)}$ and the reson why f(A)=0 and f(B)=1. And I cannot understand the link between the conclusion and the original goal of proving the space is normal. $\endgroup$ – Pedro Gomes Nov 10 '18 at 16:19

To give another proof more along the lines you started out with, using unions of open balls centered at the points of the sets:

Suppose we have $A,B$ disjoint and closed in a metric space $(X, d)$.

For each $a \in A$ we have that we have $r_a>0$ such that the open ball $B(a, r_a)$ is disjoint from $B$. This follows from $B$ being closed and $a \notin B$. So for these $r_A$ we have the property $$\forall a \in A : \forall b \in B: d(a,b) \ge r_a\tag{a}$$

Likewise for each $b \in B$ we have some $s_b>0$ such that $B(b,s_b) \cap A=\emptyset$ or

$$\forall b \in B: \forall a\in A: d(b,a) \ge s_b\tag{b}$$

Now define $O_A=\bigcup\{B(a,\frac{r_a}{2}): a \in A\}$ and $O_B=\bigcup\{B(b, \frac{s_b}{2}): b \in B\}$, which are both open as unions of (basic) open balls. Clearly $A \subseteq O_A$ and $B \subseteq O_B$.

Suppose that $p \in O_A \cap O_B$, which means that for some $a_0 \in A$ and some $b_0 \in B$ we have that $p \in B(a_0, \frac{r_{a_0}}{2})$ and $p \in B(b_0, \frac{s_{b_0}}{2})$. Define $r=\max(r_{a_0}, s_{b_0})$, and by $(a)$ and $(b)$ we have that $r \le d(a_0, b_0)$,

But then: $$r \le d(a_0, b_0) \le d(a_0,p)+ d(p, b_0) < \frac{r_{a_0}}{2} + \frac{s_{b_0}}{2} \le \frac{r}{2} + \frac{r}{2}=r$$

We now have the contradiction $r < r$ so the intersection of $O_A$ and $O_B$ is empty and we have shown normality.


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