Goldbach's conjecture is one of the oldest unsolved problems in number theory and in all of mathematics.
4 = 2 + 2 6 = 3 + 3 8 = 3 + 5 10 = 3 + 7 = 5 + 5 12 = 5 + 7 14 = 3 + 11 = 7 + 7 etc.
In 1742, the Prussian mathematician Christian Goldbach wrote a letter to Leonhard Euler in which he proposed the following conjecture:
Every integer greater than 2 can be written as the sum of three primes.
He considered 1 to be a prime number, a convention subsequently abandoned. So today, Goldbach's original conjecture would be written:
Every integer greater than 5 can be written as the sum of three primes.
Euler, becoming interested in the problem, answered with an equivalent version of the conjecture:
Every even number greater than 2 can be written as the sum of two primes,
adding that he regarded this a fully certain theorem ("ein ganz gewisses Theorema"), in spite of his being unable to prove it.
The former conjecture is today known as the "ternary" Goldbach conjecture, the latter as the "strong" or "binary" Goldbach conjecture. The conjecture that all odd numbers greater than 9 are the sum of three odd primes is called the "weak" Goldbach conjecture. Both questions have remained unsolved ever since, although the weak form of the conjecture is much closer to resolution than the strong one.
The majority of mathematicians believe the conjecture (in both the weak and strong forms) to be true, at least for sufficiently large integers, mostly based on statistical considerations focusing on the probabilistic distribution of prime numbers: the bigger the number, the more ways there are available for that number to be represented as the sum of two or three other numbers, and the more "likely" it becomes that at least one of these representations consists entirely of primes.
A very crude version of the heuristic probabilistic argument (for the strong form of the Goldbach conjecture) is as follows. The Prime Number Theorem asserts that an integer selected at random has roughly a chance of being prime. Thus if is a large even integer and is a number between 3 and , then one might expect the probability of and simultaneously being prime to be . This heuristic is non-rigorous for a number of reasons: for instance, it assumes that the events that and are prime are statistically independent of each other. Nevertheless, if one pursues this heuristic, one might expect the total number of ways to write a large even integer as the sum of two odd primes to be roughly
Since this quantity goes to infinity as increases, we expect that every large even integer has not just one representation as the sum of two primes, but in fact has very many such representations.
The above heuristic argument is actually somewhat inaccurate, because it ignores some correlations between the likelihood of and being prime. For instance, if is odd then is also odd, and if is even, then is even, a non-trivial relation because (besides 2) only odd numbers can be prime. Similarly, if is divisible by 3, and was already a prime distinct from 3, then would also be coprime to 3 and thus be slightly more likely to be prime than a general number. Pursuing this type of analysis more carefully, Hardy and Littlewood in 1923 conjectured (as part of their famous Hardy-Littlewood prime tuple conjecture) that for any fixed , the number of representations of a large integer as the sum of primes with should be asymptotically equal to
where the product is over all primes , and is the number of solutions to the equation in modular arithmetic, subject to the constraints. This formula has been rigorously proven to be asymptotically valid for from the work of Vinogradov, but is still only a conjecture when . In the latter case, the above formula simplifies to 0 when is odd and to
when is even, where is the twin primes contant,
This asymptotic is sometimes known as the extended Goldbach conjecture. The strong Goldbach conjecture is in fact very similar to the Twin Prime Conjecture, and the two conjectures are believed to be of roughly comparable difficulty.