The statement phrased by Christian Goldbach is a “conjecture”, so, as a matter of principle, it is an hypothesis. This means that it can be:
- True, that is all even numbers greater than 2 can be expressed as a sum of two prime numbers;
- False, that is at least one even number greater than two exists, which cannot be written as a sum of two prime numbers.
Both hypotheses are open, and scholars from all over the world are following different paths for finding a solution to what has actually become an enigma. For example, there are several proof attempts that were not successful because of problems in the reasoning.
To make matters worse, the Goldbach’s conjecture entails a numerical problem: if it’s true that an even number is a sum of two prime numbers, what are these two numbers? Determine them is rather easy if the starting number is small, but the more it grows, the more finding these two prime numbers becomes in turn “a challenge in the challenge”. Finding a solution for this numerical problem has in turn some theoretical implications: if a formula for computing them was found, and if it would work for any even number, the proofs would follow automatically.
The hunt, so, is more open than ever.
Attempts to decompose even numbers
In order to try to prove that an even number can be written as a sum of two prime numbers, one possibility is to find a way to write it first of all as a sum, and then reduce the characteristics of the related addends, until proving that two of them are enough and they are both prime: in this way, the solution is approached gradually, narrowing things down step by step. The results in this sense are many, and each of them offers a partial solution to the original conjecture, stating that an even number greater than two is certainly a sum of two prime numbers and of other numbers:
- Viggo Brun, Norwegian, in his book Le crible d’Eratosthène et le théorème de Goldbach published in 1919, proved that every even number greater than two can be written as a sum of two numbers, both of which are the product of no more than 9 prime numbers.
- The Soviet mathematician Yuri Vladimirovič Linnik has proved, in 1951, that there exist two constants h and k such that all even numbers greater than h are expressible as a sum of two odd prime numbers and of k powers of 2. A successive work, by Roger Heath-Brown and Jan-Christoph Schlage-Puchta, has proved that k is equal to 13.
- The peruvian mathematician Harald Andrés Helfgott proved, in 2013, the “weak” Goldbach’s conjecture, which states that every odd number greater than 5 can be expressed as a sum of three prime numbers. The direct consequence is that every even number greater than 2 can be written as a sum of 4 prime numbers.
To learn more:
- The proof of the weak Goldbach’s conjecture published by Helfgott
- Linnik’s proof (in Russian)
- Brown and Puchta’s paper
- Mauro Fiorentini’s page, with further information about these results
Chen and Yamada’s theorems
The result which get most close to the proof of the Goldbach’s conjecture was obtained by a Chinese mathematician, Chen Jingrun. Born in 1933 in Fujian, a district of south-eastern China, lived his youth while the Chinese empire was becoming the People’s Republic we know today. Always science enthustiast, when his math teacher at high school talked to the students about Goldbach’s conjecture, our Chen decided to try to prove it. He dedicated several years at studying it until, in 1966, using Sieve theory as a basis for his argument, wrote a paper, On the representation of a large even integer as the sum of a prime and a product of at most two primes, in which he described the most important result he had achieved, which the rest of the world would later refer to as Chen’s theorem:
Every sufficiently large even number is a sum of two prime numbers, or of a prime and a semiprime (that is the product of two prime numbers).
In arithmetical terms, the theorem can be rephrased in this manner:
There exists an even number k > 0 such that, for every even number n > k, we have that n = a + bc, where a and b are prime numbers, and c is 1 or a prime number.
The original theorem states that k exists, but it does not indicates its value. An important result in this sense is due to a Japanese mathematician, Tomohiro Yamada, of the Osaka university, who proved that k = ee36, where e is the known Napier’s constant. Una curiosità: e is also called Euler’s number… the same Euler who Christian Goldbach talked to about his conjecture for the first time.
To learn more:
Study of the counterexample set
In order to understand if Goldbach’s conjecture is true, two methods are usually followed: a proof is attempted, or a refutation. An alternate tactic, besides choosing a method or the other, consists in adopting a more direct approach: trying to quantify, to the total, the “counterexamples”, i.e. the even numbers which cannot be written as a sum of two prime numbers. In this way, depending on the result, a conclusion can be directly reached: the conjecture is true if there aren’t counterexamples, otherwise, if almost one exists, it’s false.
Attempts in this way are of different types:
- Three mathematicians, Soviet Nikolai Chudakov, Dutch Johannes van der Corput and German Theodor Estermann, between 1937 and 1938, have proved that the fraction of numbers satisfying the conjecture tends to 1, that is almost even numbers can be written as the sum of two primes: this result, then, implies that the even numbers not respecting the conjecture, if existing, are very few. The three proofs, reached independently by each scholar, are based on the studies by another Soviet mathematician, Ivan Matveevič Vinogradov.
- Previous result has been refined later in 1975, by Hugh Montgomery from USA, and Robert Charles “Bob” Vaughan from UK, who determined that, given N, the quantity of even numbers less than N not satisfying the conjecture is less than CN1-c, where c and C are two constants greater then zero.
- Wen Chao Lu from China proved the number of counterexamples less than x is much less x0,879, improving a similar result obtained by already mentioned Chen Jingrun.
To learn more:
- Chudakov’s proof (in Russian)
- Van Der Corput’s proof (in French)
- Montgomery and Vaughan’s proof
- Wen Chao Lu’s proof
Along with the theoretical way, Goldbach’s conjecture can be also studied by numerical methods, trying to find the two prime numbers which every even number is the sum of. In this way, we can study how these numbers behave, if they follow some rule, and so on.
Professor Tomás Oliveira e Silva, of Aveiro University in Portugal, followed exactly this way: he started a project, ended in 2013, for realizing an authomatic method for writing as a sum of two prime numbers all even numbers up to 4 x 10 18. Resources were the main problem: as the number to be scomposed increased, the necessary computations required more and more time and computing power from computers. In order to be successful in completing the work in reasonable time, above all in the final part in which the numbers involved were very big, the contribution of an Italian, Silvio Pardi, of the Naples location of the National Institute of Nuclear Phisics, was essential.
In this way, it was possible to state that Goldbach’s conjecture is certainly true for all even numbers up to 4,000,000,000,000,000,000, four billions of billions. It’s a very big number, not enough by itself for proving that the conjecture is always true, however still representing a certain proof that, if an even number not satisfying the conjecture exists, it would be surely bigger than that.
To learn more: