I am trying to understand the notion of extremally disconnected space (in other words Stonean space), i.e. a space in which any open set has an open closure. Could you help me and give (reasonable) examples of such spaces except discrete space?

Thanks in advance.


2 Answers 2


Maybe its to late for You but this might be helpful for someone else

A topological space $K$ is said to be extremally disconnected if $ \bar U$ is open for every open $U ⊆ K$; equivalently, in $K$ the following separation axiom is satisfied:

If $U, V ⊆ K$ are open and $U ∩ V = \emptyset $ then $\bar U ∩ \bar V = ∅$.

This separation is extremally (sic!) storng, and unusual, I can't think of any natural examples that do not come from Stone spaces of boolean algebras.

By an ultrafilter $ \mathcal F$ in boolean algebra $ \mathfrak A = (A,0,1, -, \vee, \wedge)$ we call a subset of $A$ such that:

  • $1 \in \mathcal F $ and $ 0 \not \in \mathcal F$
  • $a, b \in \mathcal F $ then $a \wedge b \in \mathcal F$
  • $a \in \mathcal F$, $b \geq a$ then $ b \in \mathcal F$
  • for every $a \in A$ either $a \in \mathcal F$ or $ -a \in \mathcal F$

Assuming AC every boolean algebra has an ultrafilter. A Stone space $K$ of boolean algebra $\mathfrak A$ is a set of all ultrafilters of $\mathfrak A$ called $\mathrm{Ult } \ \mathfrak A $ with topology generated by base set of a form $ \hat a$, $ a \in A$ where: $$ \hat a = \{ \mathcal F \in \mathrm{Ult} \ \mathfrak A : \ a \in \mathcal F \}$$

Theorem The space $K$ is a compact, totally disconnected topological space. Observe that it from this disconnectedness and compactness with have zero-dimensionality.

The mapping $a \mapsto \hat a$ is boolean isomorphism between $ \mathfrak A$ and algebra of a clopen sets in $K$.

Theorem The algebra $\mathfrak A$ is complete (every subfamily has a least upperbound) iff $K$ is extremally disconnected.

Example Consider algebra $ \mathfrak A = \mathcal P (\mathbb N )$ with natural operations. It is complete, a subset $\mathcal S$ has an upper bound $ \bigcup \mathcal S$. Hence $K_{\mathfrak A} = \beta \mathbb N$ (wiki) is extremally disconnected.

Example Let $\mathfrak B$ be the the measure algebra, i.e. the family of all equivalence classes $[B], B ∈ Bor[0, 1]$ where: $$[B] = \{ A ∈ Bor[0, 1] : λ(A \Delta B) = 0\}$$ This is again complete, by taking the sum and assuring that it is countable one by some approximation theorems. So its Stone space $K_{\mathfrak B}$ is extremally disconnected.

Remark One can check that $\ell^\infty$ is isometric to $C ( \beta \mathbb N)$ and $L^\infty [0, 1]$ is isometric to $C(K_{\mathfrak B})$.

Someone objected what this topic has to do with operator theory. But the notion of extremal disconnectedness arises naturally in functional analysis:

Theorem Space K is an extremally disconnected compact space iff $C(K)$ is an order complete Banach lattice, i.e. for every bounded from above family $Φ ⊆ C(K)$ there is an least upper bound in $C(K)$.

A Banach space $X$ is 1-injective if for every Banach space $F$ and its subspace $E$ every bounded operator $T : E → X$ has an extension to a bounded operator $ \bar T : F → X$ such that $ \| \bar T \| = \| T \|$.

Example Space $\mathbb R$ is 1-injective, which is exactly Hahn-Banach theorem.

Example Space $\ell^\infty$ is 1-injective by using Hahn-Banach for every coordinate.

Theorem [Kelly] A Banach space $X$ is 1-injective iff it is isometric to $C(K)$ space with $K$ extremally disconnected.

Corollary Space $L^\infty [0,1]$ is 1-injective.

Notion of 1-injectivity is important in isomorphic theory of seperable Banach spaces, and extremal disconectedness gives us some characterisation of this property.

Reference: Lecture notes on combinatorics in Banach spaces

  • $\begingroup$ Usage of sic in "extremally (sic!) storng" suggests that you are quoting from somewhere, but it is unclear which part of your post is a quote or where the quote is taken from. $\endgroup$ Nov 14, 2015 at 12:18
  • $\begingroup$ Good eye. ;-) This answer is partially taken from my lecture notes on a course 'combinatorics in Banach spaces' given by professor Grzegorz Plebanek. $\endgroup$
    – wroobell
    Nov 14, 2015 at 13:18
  • $\begingroup$ The natural next question is whether the notes are available online = since that might be useful addition to post. But I guess that if they are available somewhere, it will probably be only in Polish. $\endgroup$ Nov 14, 2015 at 13:24
  • 1
    $\begingroup$ Actually they are available in English :) Not exactly from my course, but this topic is also covered. Some theorems are taken exactly from there. I edited my answer. Sorry for not giving references at first. I am still not used to answering rather than asking questions. :p $\endgroup$
    – wroobell
    Nov 14, 2015 at 13:46
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    $\begingroup$ Thanks for adding the link. I think that it improves the answer in several ways: 1) If part of your answer is quote from somewhere, it is a good practice to include reference. 2) If somebody wants to see proofs and further references for theorems you listed, they can find them in the notes. 3) There is a lot of other interesting material there, so this link might be interesting not only in connection with the original question. $\endgroup$ Nov 14, 2015 at 14:42

Two simple examples of extremally disconnected spaces are $\mathbb{N}$ (or any infinite set) with the cofinite topology $\mathcal{T}=\{A:|\mathbb{N}\setminus A|<\aleph_0\}\cup\{\varnothing\}$ and $\mathbb{R}$ with the right order topology $\mathcal{T}=\{(a,\infty):a\in\mathbb{R}\}\cup\{\varnothing,\mathbb{R}\}$. In each, the closure of any nonempty open set is the whole space, hence open, and of course, every topological space satisfies $\overline\varnothing=\varnothing$, which is also open.

Note. Some texts such as Steen and Seebach's Counterexamples in Topology (SS) restrict the definition of extremal disconnectedness to Hausdorff spaces, which the above two are not. They are hyperconnected spaces, all of which are extremally disconnected (without the Hausdorff restriction).

The Hausdorff restriction limits the number of extremally disconnected spaces in SS to just these five:

  • $\begingroup$ Do Extremally Disconnected spaces have non trivial dense subsets? What would be an example, say, of a dense subset of the measure algebra $\{[B] : B \in Bor[0,1]\}$? Is this space Extremally Disconnected but not Totally Disconnected? $\endgroup$
    – Alberto
    Feb 17, 2021 at 21:15

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