# The error in Petrovski and Landis' proof of the 16th Hilbert problem

What was the main error in the proof of the second part of the 16th Hilbert problem by Petrovski and Landis?

Please see this related post and also the following post.. For Mathematical development around this historical problem please see this paper.

Added : According to their method, what of the following two statements are true?:

There are uniform numbers $$\tilde{H}(n)$$ such that every polynomial vector field $$X$$ of degree $$n$$ satisfies:

Statement 1) There are at most $$\tilde{H}(n)$$ real limit cycles of $$X$$ which lie on the same leaf.

statement 2) There are at most $$\tilde{H}(n)$$ distinct complex leaves which contains real limit cycles.

By "Leaf" I mean the leaf of the corresponding complex singular foliation of $$\mathbb{C}P^{2}$$. Some technical and historical aspects of these foliations are explained here. However in this linked paper there is no an explicit explianation about the "error".

According to the video of lecture of Ilyashenko, provided in the answer to this question by Andrey Gogolev, we ask:

What is the fate of the "persistence problem" which is mentioned by Ilyashenko? How can it be revised to become a true statement?

According to the first page of the english version of the paper of Petrovski_Landis we ask

"How they assume that a solution of the equation can be considered as an entire map from $$\mathbb{C}$$ to $$\mathbb{C}P^{2}$$? Can every leaf be parametrized by an entire map? Does this "entire-assumption" play an important role in their proof? Please See this related post.

According to comments and answers to this question, we undrestand there is no a written paper which explains the error, explicitly.Why really this is the case?

• You may find Henryk Zoladek's discussion of Hilbert's 16th problem of interest: it contains a detailed discussion of Songling Shi's demonstration that $H(2)\geq 4$ (contradicting Petrovski & Landis's claim that $H(2)=3$). Shi's Chinese 1979 paper has been translated in English, and is online. Commented Feb 3, 2020 at 21:30

Not an answer, but way too long for a comment:

According to Ilyashenko ("Centennial history of Hilbert's 16th problem," http://www.ams.org/journals/bull/2002-39-03/S0273-0979-02-00946-1/S0273-0979-02-00946-1.pdf), the claimed result of Petrovski and Landis was disproved by Ilyashenko and Novikov (pg. 303). A citation to this disproof is not given, Ilyashenko's bibliography lists no articles by Ilyashenko & Novikov, and I can't find a paper by Ilyashenko and Novikov on the subject, so I don't believe they ever published their argument; however, Ilyashenko does point out that Landis and Petrovski claimed $H(2)=3$, but Shi Song Lin and Chen & Wang independently constructed quadratic vector fields with four limit cycles, showing that $H(2)\ge 4$. Their methods were based on Poincare Bendixon theorem on $S^{2}$ and the order of weak focus of quadratic system.

• So what was the error?

This seems annoyingly hard to find! Landis and Petrovski's paper "On the number of limit cycles of the equation ${dy\over dx}={P(x, y)\over Q(x, y)}$, where $P$ and $Q$ are polynomials of 2nd degree" is freely available online (http://www.mathnet.ru/links/b7f25c4ee0acc2e5f39e5614ea6e4c54/sm5216.pdf); unfortunately, it seems to only be available in Russian, which I can't read. Additionally, I can't find a copy of Chen & Wang's paper or of Shi's paper online (although the abstract of the latter is available), which I would suspect say at least a little bit about Landis and Petrovski's arguments.

On the plus side, explicit constructions of quadratic vector fields with four limit cycles - even really nice ones! - are available in English: e.g., http://arxiv.org/pdf/1002.1055v1.pdf. I can't seem to find the simplest examples, though, which would presumably be the nicest for trying to figure out (without access to the paper itself) what the error was.

As a side question,

• When was it noticed?

Ilyashenko says he and Novikov found the error "in the early 60s" (pg. 303). Backing this up, Shi's abstract mentions that the question "Is there a quadratic vector field with exactly 4 limit cycles?" was asked at a 1974 symposium on Hilbert's problems, so presumably the error was already known (Landis & Petrovski published in 1955). Then things get a bit tricky for me to track down: presumably, the symposium in question was "Mathematical developments arising from Hilbert's problems" (http://www.ams.org/bookstore-getitem/item=PSPUM-28); however, the table of contents for the proceedings of that symposium show no talk or paper on the 16th problem. Thus, although I'd imagine more specific discussion of when the error was noticed (and maybe even what it was!) would have happened at this symposium, I can't find it.

Beyond that, I have no idea when exactly the error was discovered.

• It might be worth mentioning Landis, E. M.; Petrovskiĭ, I. G., A letter to the editors (Russian), Mat. Sb. (N.S.) 73 (115) 1967 160, MR0210979 (35 #1864), in which the authors acknowledge an error in the proof of Lemma 12 in their 1955 paper. The original Landis-Petrovsky paper in English translation in Petrovskiĭ, I. G.; Landis, E. M., On the number of limit cycles of the equation dy/dx=P(x,y)/Q(x,y), where P and Q are polynomials of the second degree, 1958 Amer. Math. Soc. Translations, Ser. 2, Vol. 10 pp. 177–221, American Mathematical Society, Providence, R.I., MR0094521 (20 #1036). Commented Jun 17, 2014 at 4:38
• Another source that might be worth a look is Ilyashenko and Yakovenko, eds., Concerning the Hilbert 16th Problem, Translations AMS 2:165 (1995). Commented Jun 17, 2014 at 4:39
• @GerryMyerson Thank you very much for your comment about the error in lemma 12 in their paper. Commented Jun 17, 2014 at 14:26
• @Noah Thanks for your very interesting answer. I remember that I read a small part of their english paper, befor my graduation. I remember an statement in the paper" if two limit cycles on the same leaf are homologe, then the leaf is an algebraic leaf and a generic algebraic equation does not have an algebraic solution..." Commented Jun 17, 2014 at 14:32
• According to an interview of Ilyashenko, he found the flaw in the spring 1963. A few months later, Novikov was giving a series of talks on Petrovski-Landis' proof in Gelfand's seminar, and Ilyashenko told him about the mistake. Novikov later responded to some attempts by Landis to save the proof, and he was mentioned in the withdrawal letter as the one who found the flaw, "because Novikov was a renowned scholar and I was merely a grad student, so it would be awkward to put both names in the letter" Commented Aug 10, 2014 at 17:21

Ilyashenko explaines very well the strategy and the main error of Petrovski-Landis in this lecture:

http://www.mathnet.ru/php/seminars.phtml?option_lang=rus&presentid=8316

Watch from 50m.

• thank you very much for the video. There is a problem in my download system. Can I save it in my computer after downloading? Did you watched the part of Petrovski Landis strategy?can you sketch this strategy and ITS Main ERROR? Do you know what is the true version of their strategy?My deep thanks to you for the video. Commented Jul 12, 2014 at 0:20

• Q1 (the first yellow boxed question in the OP):

The current status of the "persistence problem" has been discussed by Ilyashenko in Complex length and persistence of limit cycles in a neighborhood of a hyperbolic polycycle (2014), see also Persistence Theorems and Simultaneous Uniformization (2006).

As discussed on page 285 of the 2014 paper, the persistence theorem introduced by Petrovskiĭ and Landis (P&L), without a valid proof, asks whether the real limit cycle of a real planar polynomial vector field persists as a complex limit cycle for the complexification of the corresponding differential equation. Ilyashenko notes that the theorem is still unproven for the general family of polynomial equations, but does give a proof for a particular case (and references to other special cases).

In general, the persistence theorem applies if a complex singular foliation of $${\mathbb C}{\mathbb P}^2$$ satisfies a certain condition of "simultaneous uniformization", explained in the 2006 paper. The error in Petrovskiĭ & Landis amounts to the absence of a proof that polynomial differential equations in the plane are simultaneously uniformizable.
The 2006 paper also goes some way towards an answer of the request in the OP for "a written paper which explains the error in P&L".

In connection with this recent MO question asking for Examples of incorrect arguments being fertilizer for good mathematics?, the P&L paper is a good example, as also pointed out here.

historical note: according to V.Gaiko, the error in Petrovskiĭ and Landis's proof was first noted by the Belorussian mathematician N.P. Erugin.

• Thank you very much for your answer. I am sorry I forget to give its bounty on time. I just remedy with consideration of more 500 bounty. Commented Feb 10, 2020 at 16:28
• any pointers to what else you would want to know about the P&L paper? Commented Feb 10, 2020 at 19:32
• Thank you again for your very interesting answer. The reason for another bounty was that I forgot to give it ontime so I remedy my fault. Commented Feb 11, 2020 at 19:17
• this is superbly generous... Commented Feb 11, 2020 at 20:56