A well known urban legend states that Cauchy's last words to the Academy where:

C'est ce que j'expliquerai plus au long dans un prochain mémoire. ("I will explain it in greater detail in my next memoire.")

I've always wondered, never managing to figure it out, what was to be explained in his next memoire. Is there any guess about what the new theory to be unveiled consisted off?

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    $\begingroup$ You are the first to get the Student Badge. $\endgroup$ Commented Oct 28, 2014 at 20:45
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    $\begingroup$ I think the only safe answer here is that we don't really know. But maybe someone who knows more about Cauchy's life than me can make some educated guess. $\endgroup$
    – Logan M
    Commented Oct 29, 2014 at 21:50

1 Answer 1


Of course, the phrasing "This is what I shall explain..." implies that Cauchy has just stated what this theorem is, so it would seem that yes, we should have a very good chance at finding out what it was, provided we can find out where the statement is from.

It appears to be from a note published on the 4th of May 1857 called Sur l'utilisation des régulateurs en Astronomie. This is the last article in the last volume of "Série 1" of Cauchy's collected Oeuvres, excluding what appears to be the minutes of Académie on the day Cauchy's death was announced, and assuming the date under the article's title is the publication date, it appeared just a few weeks before Cauchy's death. Thus it appears likely that this was indeed his last publication.

It's article 589 in Tome XII, Série 1 of Œuvres Complètes d'Augustin Cauchy, page 455, an online copy of which can be found here.

The note seems to be about the use "Régulateurs" in astronomy, where I assume a "Régulateur" is an astronomical regulator, an old type of pendulum clock used in observatories.

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Translation (my own, and although I'm fluent in french I'm less fluent in 19th century astronomy, and without knowing what exactly Cauchy is talking about I have no idea what the correct english technical terms are - I've therefore simply translated them as literally as possible):

On the use of regulators in Astronomy

I noted in the previous session the advantages of using regulators in mathematical Analysis. I will add that not only the variables, but indeed the parameters contained in the given equations, finite or differential, or even partial differential, can be supposed to be developped according to the ascending powers of a given regulator. In many problems, particularly in Astronomy, this observation will allow us to render monodrome or monogeneous the variations of the various orders of unknowns which are expanded as series following the increasing powers of a single given regulator. This solves the question raised in my previous Mémoire, on the possibility of developping the coordinates which determine the orbits of the planets about the Sun, or of satellites about the planets, according to the ascending and descending powers of the trigonometric exponentials whose arguments are the excentric or average anomalies, and thereby according to the ascending or descending powers powers of the keys of the orbits. This I will explain in more detail in a future Mémoire.

I frankly have no idea what any of the above means. The article is followed by the aforementioned bit of text about Cauchy's death.

  • $\begingroup$ From my reading, it appears that "régulateurs" haven't got much to do with the astronomic clock, but rather to the tool described here: institut.math.jussieu.fr/theses/2006/riou/these-riou.pdf . I'll study this more in-depth when I can find the time to do so. It's got to do with the "méthode des petites peturbations", which is very usefull in Astronomy so I believe we've got our culprit. $\endgroup$
    – VicAche
    Commented Nov 1, 2014 at 10:58
  • $\begingroup$ @VicAche It's completely possible - I only thought of the clock because it turned up when I put "régulateur astronomie" into Google. If you do want to know more, another useful line of research would be to find out what was said at/in the "previous session" Cauchy mentioned. $\endgroup$
    – Jack M
    Commented Nov 1, 2014 at 11:10
  • $\begingroup$ I checked and I believe I'm right there. The method of small perturbations is used to describe the impact of a small-but-periodical factor on the solution of an otherwise solved problem. We use it pretty commonly, so I was surprised to learn it was actually such a late development. Thank you very much for your research! ( en.wikipedia.org/wiki/Perturbation_theory ) $\endgroup$
    – VicAche
    Commented Nov 1, 2014 at 11:16

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