Origins of the two Goldbach’s conjectures

Background

Europe, 18th century. While the Western powers were all a flourishing of industries, cultural exchanges and scientific discoveries, the Russian Empire was always one step behind, with an industry that was little more than artisanship, a feudal economy, and a public education that wasn’t even worthy of the name.
But the new Tsar Peter the Great, after touring Europe in search of allies against the Turks who threatened the southern borders, was able to see this abysmal gap with his own eyes, and decided that the time had come for a turning point. He immediately started a great reform campaign, which turned his empire upside down like a sock: he reorganized the state administration in order to make it more efficient, financed the creation of new industries, instituted a new school system, and more. He also founded new commercial hubs, such as Saint Petersburg, built at the mouth of the Neva River on a territory torn from the Swedes after years of war: from a small military fortress, in a swampy area on which no one would have bet a penny, drained and consolidated by the hard work of laborers, master builders and architects, a grandiose and modern city arose, elected as the new capital of the empire.
From the beginning it was clear that the price to be paid would be higher than one could imagine, but progress was a goal to be achieved at all costs, because the alternative would be to see the empire dissolve and get lost in oblivion.
There was also another serious problem to be addressed, which was the real heart of the matter: in the rest of Europe, scientific and technological research was galloping like never before, while in the Russian Empire they had stopped for decades. For this reason, in 1724, the Tsar gave orders to his personal doctor, Lorenz Blumentrost, to found a special institution, to make it a springboard towards the future. It was baptized the Saint Petersburg Academy of Sciences, named after the city where it was built.

Outer façade and inner view of the Kunstkamera, first headquarters of the Saint Petersburg Academy of Sciences; engraving by Grigory Anikiyevich Kachalov, 1740
Outer façade and inner view of the Kunstkamera, first headquarters of the Saint Petersburg Academy of Sciences; engraving by Grigory Anikiyevich Kachalov, 1740

But the Academy was not born to be a university like any other: its purpose was not only education, but also and above all research, because it was necessary to progress, and it should be done quickly.
Many brilliant minds had the honor of participating in the life of the Academy, joining it as professors and researchers; among them there was one of the two protagonists of our story, a Prussian mathematician named Christian Goldbach, who had already made himself known in the scientific community for Specimen methodi ad summas serierum, an important treatise about numerical series. He sent an application for admission to the Academy in 1725, which was not accepted immediately, but earned him the chair of mathematics, as well as a role as a historian; he also had the honour of attending the opening ceremony with the title of Glavnyy uchenyy sekretar ‘Prezidiuma (general scientific secretary of the Presidium ), which he also retained later.

A long collaboration

The following year, a serious mourning suddenly struck the Academy: Nicolaus II Bernoulli, professor of mathematics and physics, was struck down by a violent attack of fever. This dramatic event caused a chain reaction: his place was taken by his brother Daniel, who however had to leave his chair of mathematics and mechanics applied to physiology, which then, in turn, lost its teacher. This was a problem, but Daniel already had in mind the perfect name for his successor: a family friend, in whom his father Johann, also a mathematician, had already discovered a great talent for numbers as a child. His name was Leonhard Euler, one of the most important mathematicians who ever existed, who will have a fundamental role in the birth of the Conjecture, as we will see shortly.
Euler accepted the proposal, but initially tried to stall, because in the meantime he was trying to get a chair in physics at the University of Basel, his hometown; in May 1727, when it became clear to him that the attempt was failing, Euler left for Saint Petersburg. Arrived in the city, he took part in the funeral of Catherine I, second wife of Peter the Great and regent of the empire, and then went to a reception, organized by Blumentrost, who became the first president of the Academy: it was here that Euler and Goldbach met for the first time.
Later, there was no lack of opportunities to meet again and also to work together, during lectures, demonstrations of experiments, and more generally in various events in the life of the Academy. It was more like a working relationship between colleagues, but they soon discovered they had something in common: an interest in number theory.
The following year, Goldbach received a proposal that no one would say no to: he was appointed tutor of the young Peter II, who succeeded Catherine as the new tsar, being the only male heir of Peter the Great, who was his grandfather. In order to carry out such an important task, Goldbach moved to Moscow, which was made once again the capital of the empire; therefore, in order to continue having contact with the Academy, he had to use the post, which was the fastest mean of communication existing at the time. He wrote letters to several professors, but the correspondence with Euler became more frequent, since the latter, after reading his paper De terminis generalibus serierum about numerical series, wrote him a letter to share with him some new results he got about the subject; since then the letters, written in German and Latin, began increasingly to focus about approximation, factorization and prime numbers. Many of these letters have come down to us, but not all of them, because some have been lost.
For a while, communication by letters was no longer needed, since, when Peter II died prematurely in 1732, the new empress Anna Ivanovna Romanova moved the capital back to Saint Petersburg, so Goldbach returned to live there and attend the Academy in person, and then see each other with Euler; during this period they often collaborated in various activities of the institution, the most important of which was taking part in a commission, set up by the new president Karl Hermann von Brevern, in order to implement a reform of the finances.
This coexistence continued until 1741, when Euler accepted the invitation of Frederick II, King of Prussia, who offered him a place at the Berlin Academy of Sciences, so, from that moment, the flow of letters was resumed.

Birth of two mysteries

As time passed, the relationship between Goldbach and Euler consolidated, to the point that, when Euler had his first child in 1734, he wanted Goldbach to be his godfather along with the then president of the Academy, Johann Albrecht von Korff. When afterwards in 1738 Euler, following a serious illness that almost cost him his life, lost sight in his right eye, Goldbach took care of talking about it with the president, in order to dispense him from one of his duties, the examination of geographic maps, as part of a project aimed to create a general map of the empire. Both were always very busy, Goldbach because of his duties as Academy secretary, Euler because he was always engeged in solving mathematical problems that had remained unsolved for decades, but they always found the time to carry out their joint research. It involved both insights into the works of other colleagues and new possible theorems; in the letter dated 7 June 1742, for example, Goldbach wrote:

I’d like to risk another conjecture of that kind: any number composed from two primes is the sum of as many prime numbers (including the unit) as one wishes, right down to the sum that consists just of ones(*). For example:
4 = \begin{cases} 1 + 3 \\ 1 + 1 + 2 \\ 1 + 1 + 1 + 1 \end{cases} 5 = \begin{cases} 2 + 3 \\ 1 + 1 + 3 \\ 1 + 1 + 1 + 2 \\ 1 + 1 + 1 + 1 + 1 \end{cases} 6 = \begin{cases} 1 + 5 \\ 1 + 2 + 3 \\ 1 + 1 + 1 + 3 \\ 1 + 1 + 1 + 1 + 2 \\ 1 + 1 + 1 + 1 + 1 + 1 \end{cases}

etc.
(*) After reading this through again, I see that the conjecture can be proved quite rigorously for the case n + 1 if it holds for the case n, and if n + 1 can be split into two prime numbers. The proof is very easy.

Auf solche Weise will ich auch eine conjecture hazardiren: dass jede Zahl, welche aus zweyen numeris primis zusammengesetzt ist, ein aggregatum so vieler numerorum primorum sey, als man will (die unitatem mit dazu gerechnet), biss auf die congeriem omnium unitatum (*); zum Exempel:
4 = \begin{cases} 1 + 3 \\ 1 + 1 + 2 \\ 1 + 1 + 1 + 1 \end{cases} 5 = \begin{cases} 2 + 3 \\ 1 + 1 + 3 \\ 1 + 1 + 1 + 2 \\ 1 + 1 + 1 + 1 + 1 \end{cases} 6 = \begin{cases} 1 + 5 \\ 1 + 2 + 3 \\ 1 + 1 + 1 + 3 \\ 1 + 1 + 1 + 1 + 2 \\ 1 + 1 + 1 + 1 + 1 + 1 \end{cases}
etc.
(*) Nachdem ich dieses wieder durchgelesen, finde ich, dass sich die conjecture in summo rigore demonstriren lässet in casu n + 1, si successerit in casu n, et n + 1 dividi possit in duos numeros primos. Die demonstration ist sehr leicht.

This additive property of integers has a fairly simple proof, as Goldbach himself states, but there is a peculiarity. When he speaks of prime numbers (including the unit), he means that 1 is not only part of the set of numbers used in the decomposition, but also of the set of prime numbers, because at the time 1 was considered as such, while today it’s no longer true.
After this property, Goldbach added another sentence:

And at least it appears to be true that every number greater than 2 is the sum of three prime numbers.

Es scheinet wenigstens, dass eine jede Zahl, die grösser ist als 2, ein aggregatum trium numerorum primorum sey.

This is the first formulation of what has gone down in history as “Goldbach’s weak conjecture”, and today we know that it is not just a conjecture, but a real theorem, which has been proved.
But the investigation did not stop there, because Euler, on the 30th of the same month, sent an answer, in which he proposed some remarks:

That any number which is resolvable into two prime numbers can also be split into as many prime numbers as one wishes, can be illustrated and confirmed from an observation that you, Sir, communicated to me some time ago, when you stated that any even number is the sum of two primes. For if the proposed number n is even, it is the sum of two primes, and as n - 2 is another sum of two primes, n is also the sum of three, of four and so on. On the other hand, if n is an odd number, it certainly is a sum of three primes, as n - 1 is a sum of two, and can therefore also be resolved into as many more as one likes. Indeed, I consider the statement that any even number is the sum of two primes to be an utterly certain theorem, notwithstanding the fact that I cannot prove it.

Dass eine jegliche Zahl, welche in zwey numeros primos resolubilis ist, zugleich in quot, quis voluerit, numeros primos zertheilt werden könne, kann aus einer Observation, so Ew. vormals mit mir communicirt haben, dass nehmlich ein jeder numerus par eine summa duorum numerorum primorum sey, illustrirt und confirmirt werden. Denn, ist der numerus propositus n par, so ist er eine summa duorum numerorum primorum, und da n − 2 auch eine summa duorum numerorum primorum ist, so ist n auch eine summa trium, und auch quatuor, und so fort. Ist aber n ein numerus impar, so ist derselbe gewiss eine summa trium numerorum primorum, weil n − 1 eine summa duorum ist, und kann folglich auch in quotvis plures resolvirt werden. Dass aber ein jeder numerus par eine summa duorum primorum sey, halte ich für ein ganz gewisses Theorema, ungeachtet ich dasselbe nicht demonstriren kann.

Here Euler cites what we know today as the “strong conjecture”, but there are some important aspects:

  • Euler cites an observation that you, Sir [referred to Goldbach], communicated to me some time ago, when you stated that any even number is the sum of two primes, but we have no transcript of this communication; so, either they discussed it in person, or it’s in one of the lost letters. This letter, therefore, is currently the only documented proof that the strong conjecture was formulated by Goldbach.
  • The weak conjecture was formulated by Goldbach (in the previous letter), but its current formulation, if n is an odd number, it certainly is a sum of three primes, it is therefore an Euler’s refinement.

References

Leonhardi Euleri Opera Omnia – Leonhardi Euleri commercium epistolicum cum Christiano Goldbach

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