# What is your favorite proof of Tychonoff's Theorem?

Here is mine. It's taken from page 11 of "An Introduction To Abstract Harmonic Analysis", 1953, by Loomis:

(By the way, I don't know why this book is not more famous.)

To prove that a product $K=\prod K_i$ of compact spaces $K_i$ is compact, let $\mathcal A$ be a set of closed subsets of $K$ having the finite intersection property (FIP) --- viz. the intersection of finitely many members of $\mathcal A$ is nonempty ---, and show $\bigcap\mathcal A\not=\varnothing$ as follows.

By Zorn's Theorem, $\mathcal A$ is contained into some maximal set $\mathcal B$ of (not necessarily closed) subsets of $K$ having the FIP.

The $\pi_i(B)$, $B\in\mathcal B$, having the FIP and $K_i$ being compact, there is, for each $i$, a point $b_i$ belonging to the closure of $\pi_i(B)$ for all $B$ in $\mathcal B$, where $\pi_i$ is the $i$-th canonical projection. It suffices to check that $\mathcal B$ contains the neighborhoods of $b:=(b_i)$. Indeed, this will imply that the neighborhoods of $b$ intersect all $B$ in $\mathcal B$, hence that $b$ is in the closure of $B$ for all $B$ in $\mathcal B$, and thus in $A$ for all $A$ in $\mathcal A$.

For each $i$ pick a neighborhood $N_i$ of $b_i$ in such a way that $N_i=K_i$ for almost all $i$. In particular the product $N$ of the $N_i$ is a neighborhood of $b$, and it is enough to verify that $N$ is in $\mathcal B$. As $N$ is the intersection of finitely many $\pi_i^{-1}(N_i)$, it even suffices, by maximality of $\mathcal B$, to prove that $\pi_i^{-1}(N_i)$ is in $\mathcal B$.

We have $N_i\cap\pi_i(B)\not=\varnothing$ for all $B$ in $\mathcal B$ (because $b_i$ is in the closure of $\pi_i(B)$), hence $\pi_i^{-1}(N_i)\cap B\not=\varnothing$ for all $B$ in $\mathcal B$, and thus $\pi_i^{-1}(N_i)\in\mathcal B$ (by maximality of $\mathcal B$).

Many people credit the general statement of Tychonoff's Theorem to Cech. But, as pointed out below by KP Hart, Tychonoff's Theorem seems to be entirely due to ... Tychonoff. This observation was already made on page 636 of

Chandler, Richard E.; Faulkner, Gary D. Hausdorff compactifications: a retrospective. Handbook of the history of general topology, Vol. 2 (San Antonio, TX, 1993), 631--667, Hist. Topol., 2, Kluwer Acad. Publ., Dordrecht, 1998

The statement is made by Tychonoff on p. 272 of "Ein Fixpunktsatz"

where he says that the proof is the same as the one he gave for a product of intervals in "Über die topologische Erweiterung von Räumen"

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Definitely, the one I like the most is the proof via ultrafilters. You only have to state the compactness of a topological space in terms of ultrafilters, which is easily obtained by the definition via open coverings (warning: the equivalence of the definitions is where one uses AC)

X is compact if and only if every ultrafilter is convergent.

Then one observes that

1. any image of an ultrafilter is an ultrafilter (in particular, any projection from a product space)

2. any filter in the product space converges if and only if all its projections converge .

You really only need a few definitions and few natural properties. My test about how nice is a proof is: can I teach it to somebody just while standing in the queue at the canteen, on into subway car?

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Dear Pietro Majer: I think Loomis's proof (the one I gave) is the same as the one you mention (due to H. Cartan if I'm not mistaken). Loomis hides the (ultra)filters. But I think he hides them very nicely. (Loomis credit the proof he gives to Bourbaki.) –  Pierre-Yves Gaillard May 30 '10 at 7:58
After a closer look at Loomis' one, I agree: in the substance it's the same as H.Cartan (it seems to me your're right with the attribution to Cartan, but I'm not certain either). But, I think here's an important issue: to give or not to give a name to the objects one uses? Sometimes there is no need at all (you know those papers with definitions of weird objects that only enter once in a proof). In this case, I think the proof greatly gains in semplicity introducing the notion of filter. I would even say, the notion of filter is the most important byproduct of the compactness theorem. –  Pietro Majer May 30 '10 at 8:28
Actually, the axiom of choice is used twice in the proof. First, you have to use it to characterize compactness by ultrafilters. For this, you do not need the full axiom of choice, the boolean prime ideal theorem suffices. The second application is in picking for each coordinate a point the projection converges to. Here you need the full axiom of choice. For Hausdorff spaces, you don't need the second part though, because than a filter cannot converge to different points. –  Michael Greinecker May 30 '10 at 8:55
I also like Cartan's ultrafilter proof best. As people have said, all the cleverness is hidden in the definition of ultrafilters. With this in hand, Tychonoff's theorem becomes a consequence of a few very straightforward facts about filters on products. As to whether it's better to hide the ultrafilters or not, I think the test should be whether ultrafilters are of any use outside of this specific context, the answer to which is of course YES! Thus I would recommend this proof even to someone who doesn't already know about ultrafilters -- learning about them is good in and of itself. –  Pete L. Clark May 30 '10 at 9:11
Dear Pietro, Pete, Spencer: In "L'intégration dans les groupes topologiques" Weil explains that his proof of the existence of a Haar measure on locally compact groups was obtained by hiding filters. I think the situations are very similar. (And I find Weil's proof incredibly beautiful - for the ideas and the style.) –  Pierre-Yves Gaillard May 30 '10 at 10:50

I like the proof from Alexander's subbase lemma. E.g. A proof here. That lemma also gives the compactness criterion in ordered spaces (completeness implies compactness).

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Thank you for your answer. I noticed that Walter Rudin gives this proof in his "Functional Analysis". –  Pierre-Yves Gaillard May 31 '10 at 4:51

My favorite proof is the one from Johnstone's Stone spaces for locales because it works without the axiom of choice.

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Tychonoff and AC are equivalent in ZF. –  Martin Brandenburg May 30 '10 at 8:07
@Martin: Tychonoff's theorem for locales does not require any form of the axiom of choice. However, the categories of spatial locales and sober topological spaces are equivalent only if AC is true. –  Dmitri Pavlov May 30 '10 at 8:39
No, the categories of spatial locales and sober spaces are always equivalent. The reason that Tychonoff for locales doesn't imply Tychonoff for spaces is that the locale product of a family of spatial locales may no longer be spatial. –  Mike Shulman May 31 '10 at 2:41
@Mike: Yes, you are right. What I actually meant is that equivalence of categories of compact regular locales and compact regular (or Hausdorff) topological spaces requires some form of AC. –  Dmitri Pavlov May 31 '10 at 8:03

Here is Tychonoff's original proof, for powers of the unit interval. He builds a complete accumulation point of a given infinite set by transfinite recursion along the index set. On page 772 of this paper one finds the formulation of the general theorem (in my translation): "The product of compact spaces is again compact. One proves this theorem word for word as in he case of the compactness of the product of intervals". Some authors (Folland, see comment below and Walter Rudin in his Functional Analysis') credit Čech with proving the general result but Čech's proof is the same as Tychonoff's and, based on a reading of his papers, I think Tychonoff deserves full credit for the theorem and its proof.

@Henno: not Fundamenta but Mathematische Annalen.

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Dear KP Hart: Thank you for your answer. (It contains at least one typo ("the the proof").) --- Folland (Real Analysis) claims that the general statement is due to Cech (On bicompact spaces, Ann. of Math. 38 (1937), 823-844. MR 1503374). What's your opinion? –  Pierre-Yves Gaillard May 30 '10 at 16:45
Walter Rudin expressed the same opinion in his Functional Analysis' but on pae 772 of the second paper that I linked to you'll find (in my translation): The product of compact spaces is again compact. This one proves word for word as in he case of the compactness of the product of intervals. I think Tychonoff deserves full credit for the theorem and its proof. The proof in \v{C}ech's paper is the same as Tychonoff's. –  KP Hart May 30 '10 at 19:01
Very interesting!!! Thank you! --- I was wondering if you couldn't insert the contents of your comment into your answer, to make them more visible. –  Pierre-Yves Gaillard May 31 '10 at 7:01
Thank you! Two more suggestions: (1) Replace "This one proves word for word as in he case" by "One proves this theorem word for word as in the case". (2) To type Cech with the caron, copy and paste "&#268;ech" (without the quotation marks). (It should work.) –  Pierre-Yves Gaillard May 31 '10 at 10:45
Here are different links to Tychonoff's articles: Über die topologische Erweiterung von Räumen (springerlink.com/content/l656352441w67612/…). Ein Fixpunktsatz (springerlink.com/content/n61706447r886l58/…). –  Pierre-Yves Gaillard May 31 '10 at 12:28

My favorite is the proof via nets by Paul Chernoff. A VERY clever use of generalized convergence in point set topology! http://www.jstor.org/pss/2324485

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Thank you! Unfortunately I don't have access to JSTOR. Chernoff's proof is also in Folland's Real Analysis. Does anybody know a public link to Chernoff's paper? –  Pierre-Yves Gaillard May 30 '10 at 7:47
@Pierre-Yves: Google "Tychonoff sets". @AnrewL: I agree, this is really the most intuitive proof. –  Martin Brandenburg May 30 '10 at 8:18
@Pierre Yikes,I forgot,that's true,Folland DOES have the proof in his text! I learned the proof from Chernoff's original paper,which was required reading in John Terilla's point set topology course.Interestingly,John learned of the proof in James Stasheff's topology course when he was a graduate student at the University Of North Carolina. –  Andrew L May 30 '10 at 15:49
This proof is also in Volker Runde's "A taste of topology". –  Bruno Stonek Jul 8 '11 at 21:55

Since all of the answers to this question(except the one involving Alexander's subbase lemma) refer to a usually strange rehashing of the ultrafilter proof (BOO), I decided to give two nice proofs to Tychonoff's theorem here for Hausdorff spaces.

The first proof of Tychonoff's theorem for Hausdorff spaces uses the Stone-Cech compactification. This proof is useful when one constructs the Stone-Cech compactification before Tychonoff's theorem.

Proof: Assume that $X_{i}$ is compact for $i\in I$. Let $X=\prod_{i\in I}X_{i}$ be the product space. Then each projection $\pi_{i}:X\rightarrow X_{i}$ extends to a continuous map $\overline{\pi_{i}}:\beta X\rightarrow X_{i}$ since each $X_{i}$ is compact. Therefore the map $f:\beta X\rightarrow X$ where $f(x_{i})_{i\in I}=(\overline{\pi_{i}}(x))_{i\in I}$ is a continuous surjection, so $X$ is compact being the continuous surjective image of $\beta X$. QED

For the second proof we use the following facts about uniform spaces that every mathematician should be aware of.

i. Every compact Hausdorff space has a unique compatible uniformity and that uniformity is complete and totally bounded.

ii. If a uniform space is complete and totally bounded, then it is compact.

Tychonoff's theorem then immediately follows from the fact that the product of complete uniform spaces is complete and that the product of totally bounded uniform spaces is totally bounded. And this proof is intuitive because it is easier to imagine that the product of complete and totally bounded uniform spaces is complete and totally bounded than to imagine that the product of compact spaces is compact.

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Is there a good reason that this answer was downvoted? Is it because these proofs only work for Hausdorff spaces or is it because it is a sin to construct the Stone-Cech compactification without Tychonoff's theorem? –  Joseph Van Name Sep 9 '13 at 14:50

The non-standard analysis proof is an interesting "application" of the ultrafilter proof: a topological space $A$ is compact if and only if every point in the associated "non-standard topological space" ${}^*A$ is near-standard, that is to say, if and only if each $x \in {}^*A$ is contained in every open neighborhood of some standard point $y \in A$ ($i.e.,$ for all $U \subset A$, $U$ open and $y \in U$ implies $x \in {}^*U \subset {}^*A$).

So let $\mathcal{X}$ be a set of topological spaces indexed by $I$, and $P$, the product of these spaces; write ${}^\*P$ for the "non-standard product" of the set ${}^*\mathcal{X}$ of topological spaces indexed by ${}^*I$, and let $x \in {}^\*P$. It suffices to show that $x$ is near-standard.

For each $\kappa \in I$, let $x_\kappa \in {}^\*X_\kappa \in {}^*\mathcal{X}$ be the $\kappa$th factor of $x$. Then $x_\kappa$ is necessarily near-standard, because $X_\kappa \in \mathcal{X}$ is compact. But this means we can find a point $y \in P$ with factors $y_\kappa \in X_\kappa$ such that $U \subset X_\kappa$ open and $y_\kappa \in U$ implies $x_\kappa \in {}^*U \subset {}^*X_\kappa$, thus $V \subset P$ open and $y \in V$ implies $x \in {}^*V \subset {}^*P$. But this means $x$ is near-standard, so $P$ is compact.

"Under the hood," this is basically the ultrafilter proof (my favorite, to answer the original question), so the axiom of choice is required in more or less the same places: while the non-standard objects exist by the Boolean prime ideal theorem, "finding" the $y_\kappa$ in non-Hausdorff spaces requires the full axiom of choice.

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How do you define the associated "non standard space" to a standard space? –  mathahada May 20 '11 at 8:42
W.A.J. Luxembourg noted an interesting consequence of doing the proof this way. If the spaces are Hausdorff, then no additional application of choice is needed. Only the use of AC that produces the nonstandard model ... and for that the Boolean Algebra Maximal Ideal Theorem (strictly weaker than AC) suffices. –  Gerald Edgar May 20 '11 at 12:17
Wait a minute, now I'm confused. I have never heard of the Boolean Algebra Maximal Ideal Theorem, but if it implies Tychonoff in this proof without AC and if Tychonoff implies AC then how can the Boolean Theorem be strictly weaker than AC? –  David White May 20 '11 at 16:03
What I'm asking I guess is: Is Tychonoff for Hausdorff spaces equivalent to AC? It seems the answer must be no or else the Boolean Theorem would be equivalent to AC –  David White May 20 '11 at 16:05
Tychonoff for Hausdorff equals Boolean principal ideal Thm. The ultrafilter proof only needs Boolean principal ideal Thm in the Hausdorff case because you dont need to choose a limit point in each factor. –  Benjamin Steinberg Feb 27 '13 at 3:38

I'm surprised that nobody has mentioned the proof using universal nets. (It can be found, e.g., in Pedersen's 'Analysis NOW' and in Bredon's 'Topology and geometry'.)

A universal net in a set X is a net which, for every $Y\subset X$, ultimately lives in $Y$ or $X\backslash Y$. One easily sees that composition of a universal net in X with a function $f:X\rightarrow Y$ gives a universal net in $Y$. Using the ultrafiler lemma, one proves that every net has a universal subnet. All this involves no topology.

Combining the above with standard facts, the proof of Tychonov is extremely short. All one needs is: - a space is compact if and only if every net has a limit point (equiv., a convergent subnet), - a net in $\prod_iX_i$ converges if and only if it converges coordinate-wise.

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This seems to be another way of "hiding" the ultrafilters in the ultrafilter proof. –  Andreas Blass Oct 12 '12 at 15:29

I have been teaching general topology for several years, but remained unsatisfied by the proofs given in the books that I based the course upon. Finally I wound up writing my own lecture notes, still not quite finished. In those notes, I give four different proofs. Two of them use (ultra)filters, but one of them avoids the terminology. The other two proofs use nets, namely Chernoff's proof without and Kelley's with universal nets.

The notes can be found at http://www.math.ru.nl/~mueger/topology2012.pdf

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I first learnt from Munkres' Topology. He gave a different motivation to use the maximal principle (Hausdorff's to be precise, but Zorn's work too) instead of the historic motivation to characterize compact spaces with a generalized version of "sequence"; i.e. filters.

What was Tychonoff's original proof? To me every proof seem to use some maximal principle; Alexander's subbase theorem also uses Zorn's lemma.

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Every proof uses the axiom of choice, because Tychonoff is equivalent to AC. For the converse, that Tychonoff's theorem implies the axiom of choice, it's hard to beat the original proof by Kelley in Fundamenta Mathematicae 37 (1950) 75--76. It may be viewed here: matwbn.icm.edu.pl/ksiazki/fm/fm37/fm3716.pdf –  John Stillwell May 30 '10 at 6:31
The original proof used the characterization (of compactness) that every infinite set has a point of complete accumulation, and involved a transfinite recursion, IIRC. It's probably in Fundamenta as well. –  Henno Brandsma May 30 '10 at 6:35
Dear John Stillwell: As a bourbakist, the expression "axiom of choice" makes no sense to me. But I thank you very much for your comment. (Unrelated aside: Thank you very much also for your wonderful translation of Dirichlet! It changed my life!) Thank you to all contributors! –  Pierre-Yves Gaillard May 30 '10 at 7:42
Dear Amadeus: I had the impression that Munkres's proof was almost the same as the Loomis's (the one I gave), which was written long before. (Thank you for correcting me if I'm wrong.) –  Pierre-Yves Gaillard May 30 '10 at 7:53
As Georges Elencwajg points out, there is a minor flaw in the original proof by Kelley that Tychonoff implies AC. Kelley uses the fact that the product of cofinite topologies is compact, a weaker statement equivalent to the theorem that every filter can be extended to an ultrafilter as shown here: math.vanderbilt.edu/~schectex/papers/kelley.pdf One can easily correct the original proof by Kelley by adding {Lambda} to the open sets, so the original proof is easily corrected. –  Michael Greinecker May 30 '10 at 13:59