nLab
Tychonoff theorem

Statement

The Tychonoff theorem is a theorem of topology, which in its classical form is equivalent to the axiom of choice. Spellings vary; the theorem is named after Андрей Николаевиц Тихонов, whose name becomes ‘Andrey Nikolayevich Tikhonov’ in modern English (BGN/PCGN) transliteration, but he originally published in German as ‘A.N. Tychonoff’.

The theorem states that a product of spaces (indexed over an arbitrary set) is compact if each of the spaces is compact. (Compare the axiom of choice: a product of sets is inhabited if each set is inhabited.)

Notice that no choice whatsoever is needed to prove the analogous theorem for locales: even in constructive mathematics, a product of locales is compact if each of the locales is compact. From that perspective, the statement equivalent to the axiom of choice is this: a product of locales is spatial if each of the locales is spatial. Either of these may be considered a Tychonoff theorem for locales.

To prove the Tychonoff theorem for Hausdorff spaces only, the full axiom of choice is not needed; the ultrafilter theorem (and possibly excluded middle) are enough.

Proof via ultrafilter convergence

One method of proof uses ultrafilter convergence. Let $⟨X_{\alpha }{⟩}_{\alpha \in A}$ be a family of compact spaces.

1. Every ultrafilter $U_{\alpha }$ on the underlying set of $X_{\alpha }$ converges to some point $x_{\alpha }$. (Consider the collection of all closed sets belonging to $U_{\alpha }$. Finite subcollections have nonempty intersection, so by compactness the intersection of the collection is also nonempty. Let $x_{\alpha }$ be a point belonging to that intersection. Every closed set disjoint from $x_{\alpha }$ belongs to the complement of $U_{\alpha }$, hence every open set containing $x_{\alpha }$ belongs to $U_{\alpha }$, i.e., $U_{\alpha }$ converges to $x_{\alpha }$.)

2. Let $U$ be an ultrafilter on ${\prod }_{\alpha }{X}_{\alpha }$. Since the set-of-ultrafilters construction $S\mapsto \beta S$ is functorial, we have a push-forward ultrafilter $U_{\alpha }=\beta ({\pi }_{\alpha })(U)$ on each $X_{\alpha }$, where ${\pi }_{\alpha }$ is a projection map. Choose a point $x_{\alpha }$ to which $U_{\alpha }$ converges. Then it is easily checked that $U$ converges to the point $⟨x_{\alpha }⟩$ in the product space.

3. Since every ultrafilter $U$ on ${\prod }_{\alpha }{X}_{\alpha }$ converges to a point, the product is compact. (If it were not, then we could find a collection of nonempty closed sets whose intersection is empty, but closed under finite intersections. This collection generates a filter which is contained in some ultrafilter $U$, by the ultrafilter theorem. Since every ultrafilter $U$ converges to some $x$, it cannot contain any closed set in the complement of $x$, hence cannot contain some closed set in the original collection, contradiction.)

   Todd: Here it’s not so clear that there’s any direct argument. For it’s at this point where we have to use the ultrafilter theorem, in order to produce a witnessing ultrafilter. Witness to what? The statement as it stands involves universal quantification over ultrafilters, so the witness would be in view of a contradiction.

Toby: Note that we use excluded middle in two places for a proof by contradiction; I'll try to remove these if possible. (Todd: I’ve rearranged the argument in the first step; the first formulation I gave looked unnecessarily “choice-y”.) We use the axiom of choice in the middle step to combine the $x_{\alpha }$ into a single family $⟨x_{\alpha }⟩$; if the $X_{\alpha }$ were Hausdorff, then the $x_{\alpha }$ would be unique, and we would not need the axiom of choice here. Finally, we use the ultrafilter theorem in the last step.

Todd: Yes, although I’m not too fussed about using indirect arguments, since in a topos, excluded middle follows from AC. I don’t know whether it also follows from a weaker choice principle equivalent to the ultrafilter theorem, but I guess I wouldn’t be surprised if it did.

Toby: As far as I know, UF and EM are independent (although I'm not certain). So I'm interested in whether the Hausdorff case requires EM as well as UF.

Todd: I wrote John Bell about this, and he kindly responded. Apparently you are right: UF (or BPIT) implies only a weakened version of EM: $\neg x\vee \neg \neg x=1$. He referred me to his paper Boolean Algebras and Distributive Lattices Treated Constructively, for this (among much else).

Toby: Great, hard results! Although I'm not sure that his ultrafilter theorem is the same as mine, since he talks about ultrafilters in arbitrary distributive lattices, while I would use a constructively stronger definition of ultrafilter in a powerset. But I should be able to figure out what carries over, now that I have a result to go for.

Proof of converses

Now we will prove that the Tychonoff theorem implies the axiom of choice, while the Tychonoff theorem for Hausdorff spaces implies the ultrafilter theorem. This is done by judicious choice of examples.

Tychonoff theorem implies axiom of choice: let $\{X_{\alpha }{\}}_{\alpha \in A}$ be a family of nonempty sets. Let $Y_{\alpha }$ be obtained by adjoining a point $p$ to $X_{\alpha }$. Topologize $Y_{\alpha }$ by taking the nontrivial open sets to be $X_{\alpha }$ and $\{p\}$. Then $Y_{\alpha }$ is compact; assuming Tychonoff, $Y={\prod }_{\alpha }{Y}_{\alpha }$ is compact. For each $\alpha$, put

$$K_{\alpha }=\{y\in Y:y_{\alpha }\in X_{\alpha }\}$$K_\alpha = \{y \in Y: y_\alpha \in X_\alpha\}

Then $K_{\alpha }$ is closed, and any finite intersection of the $K_{\alpha }$ is nonempty (use $p$ in all but finitely many components). Hence

$$\prod _{\alpha }{X}_{\alpha }=\bigcap _{\alpha }{K}_{\alpha }$$\prod_\alpha X_\alpha = \bigcap_\alpha K_\alpha

is nonempty as well, by compactness. Thus the axiom of choice follows.

Tychonoff theorem for Hausdorff spaces implies ultrafilter theorem: let $F$ be a filter on a set $X$, i.e., a filter in the Boolean algebra $PX$. An ultrafilter $U$ containing $F$ is tantamount to a Boolean algebra map $2^X\to 2$ which sends all of $F$ to 1, or equivalently to a Boolean algebra map $\varphi :B=2^X/F\to 2$. Thus it suffices to construct a Boolean algebra map $\varphi :B\to 2$ for any Boolean algebra $B$.

Let $\mid B\mid$ denote the underlying set of $B$. The set of all functions $f:\mid B\mid \to 2$ is a $\mid B\mid$-indexed product of copies of 2; considering 2 as a compact Hausdorff space, this set is compact Hausdorff, assuming the Tychonoff theorem for Hausdorff spaces. For each finite subset $S\subseteq \mid B\mid$, let $K_S$ be the set of functions $f:\mid B\mid \to 2$ for which the equations

$$f(x\wedge y)=f(x)\wedge f(y)f(\neg x)=\neg f(x)$$f(x \wedge y) = f(x) \wedge f(y) \qquad f(\neg x) = \neg f(x)

hold for all $x,y\in S$. Then $K_S$ is a closed subset of the topological space $2^{\mid B\mid }$, and it is easy (and classical) that it is nonempty (the subalgebra $B(S)$ generated by $S$ is finite and admits such an $f$, and we can take $f(b)=1$ for any $b$ outside $B(S)$). By compactness, the intersection of all the $K_S$ is nonempty, and this is precisely the set of Boolean algebra maps $\varphi :B\to 2$. Thus the ultrafilter theorem follows.

• This proof may be seen as a classical illustration of the compactness theorem for propositional logic. We may think of a Boolean algebra $B$ as a Lindenbaum algebra for a propositional theory, and a Boolean algebra map $\varphi :B\to 2$ as a model for that theory. A model exists iff a model exists for every theory generated by a finite subset of propositions, by the compactness theorem. In fact, the proof above suggests the topological provenance of the term “compactness theorem”.