# Non-separable compact space

Off the top of my head, I can't think of a non-separable compact space.

Can you provide a good example?

• That's because any compact metric space is separable. So any example has to be non metrizable – Giuseppe Negro May 7 '14 at 15:03
• – 115465 May 7 '14 at 15:07
• Thanks Léo & Giuseppe. The Alexandroff double circle is non-separable and compact. – Tom LaGatta May 7 '14 at 15:25

## 5 Answers

A not so simple, but curious example is the Alexandroff Double Circle:

Consider $C_i=\{(x,y) \in \mathbb{R}^2; x^2+y^2=i\}$ for $i=1,2$. Let $X=C_1 \cup C_2$ and $f:C_1 \to C_2$ the radial homeomorphism. Define a topology on $X$ as follows: the points in $C_2$ are all isolated; for each $x \in C_1$ and $n \in \mathbb{N}$ let $O(x,n)$ the arc in $C_1$ centered at $x$ and with length $\frac{1}{n}$. Now take $B(x,n)=O(x,n) \cup f(O(x,n)-\{x\})$ as open neighborhood. The picture below illustrate such $B(x,n)$. This way we form a basis for a topology on $X$. $X$ provided with this topology is what we call the Alexandroff Double Circle.

Now, $X$ is not separable because all points in $C_2$ are isolated, hence no countable subset of $C_2$ is dense in $C_2$. For compactness, take an open cover $\{A_{\alpha}\}$ of $X$ which consists of basis neighborhoods. Then it is also an open cover of $C_1$. But notice that the topology of $C_1$ as a subspace of $X$ is exactly the usual one. So it is compact and we can find a finite subcollection $\{A_1,\dots,A_k\} \subset \{A_\alpha\}$ that covers $C_1$. Now, by the nature of our neighborhhods, $X-(A_1\cup \dots \cup A_k)$ is a finite set, so we take an extra $A_{\alpha}$ for each of these points, obtaining a finite subcover.

I don't know if this space is useful some some other things besides counterexamples. This and a lot of other crazy spaces can be found on "Counterexamples in topology", by L. A. Steen and J. A. Seebach.

$\pi$-Base, an online version of the reference chart from Steen and Seebach's Counterexamples in Topology, gives the following examples of compact, non-separable spaces. You can view the search result for more details on these spaces.

Alexandroff Square

Closed Ordinal Space $[0, \Omega]$

Concentric Circles

Either-Or Topology

Lexicographic Ordering on the Unit Square

The Extended Long Line

Tychonoff Plank

Uncountable Excluded Point Topology

Uncountable Fort Space

Uncountable Modified Fort Space

• Awesome list, Austin! – Tom LaGatta May 8 '14 at 3:58
• The long line is my favourite example of this phenomenon! – TidSchmod Dec 15 '18 at 3:07

This I think would be the simplest example. Let $(Y,\tau_d)$ be an uncountable discrete space. Consider the space $X=Y\cup \{x\}$ with topology $\tau=\tau_d \cup \{X\}$. $(X,\tau)$ is clearly compact and, since all points in $Y\subset X$ are isolated, not separable.

The space I described is $\text{T}_0$ but not $\text{T}_1$. @Léo 's answer gives an example of a compact Hausdorff space that is not separable.

A space $X$ which is product of a collection of compact Hausdorff spaces $\{C_j\}_{j \in J}$ such that the cardinality $|J|$ of the collection is very large. $X$ is compact by Tychonoff's theorem. If $X$ is separable, i.e. if there is a countable dense subset $A$, then every point of $X$ is the limit of a sequence of points in $A$. The number of such sequences is at most the cardinality of the reals, and each sequence has at most one limit because $X$ is Hausdorff, and so $X$ has cardinality at most equal to the cardinality of the reals. But you can make $X$ have larger cardinality by choosing $|J|$ sufficiently large.

Edit: As pointed out in the comment of @Henno Brendsma, I should not have used convergence of sequences since that carries an additional assumption that each point has a countable neighborhood basis. Instead one can just use convergence of nets. The cardinality of all nets in a countable subset $A$ is bounded, so the cardinality of the set of limits of convergent nets in $A$ is bounded, and every point in the closure of $A$ is the limit of a convergent net in $A$.

• It's not true that in every point of $X$ is the limit of points from a dense subset. This is true for first countable spaces, but large products of $[0,1]$, say, won't be first countable. In fact $[0,1]^I$ is separable iff $|I| \le |\mathbb{R}|$. So your idea does work, but for a different reason. – Henno Brandsma May 7 '14 at 16:13
• The cardinality of all subnets of a countable set is not bounded. You need to use filters to get the upper bound of $2^{2^{\aleph_0}}$ for the cardinality of a separable space (because a filter on $X$ is an element of $\mathcal{P}(\mathcal{P}(X))$). This is a known area of deficiency of Moore-Smith convergence. – Robert Furber Nov 11 '19 at 10:20

Based on this question I asked recently, the one-point compactification of a non-separable metric space.

I specifically asked about the space of cadlag real-valued functions on $$[0,1]$$ under the supremum metric, but there are lots of other non-separable metric spaces out there, and other compactifications where the same argument applies.

The metric space is an open subset of its one-point compactification, and is not separable, so the one-point compactification is also not separable.