**Summary**

Goldbach conjecture in number theory is assertion (here stated in modern terms) that every even counting number greater than 2 is equal to the sum of two prime numbers. The Russian mathematician Christian Goldbach first proposed this conjecture in a letter to the Swiss mathematician Leonhard Euler in 1742. More precisely, Goldbach claimed that “every number greater than 2 is an aggregate of three prime numbers.” (In Goldbach’s day, the convention was to consider 1 a prime number, so his statement is equivalent to the modern version in which the convention is to not include 1 among the prime numbers.)

Goldbach’s conjecture was published in English mathematician Edward Waring’s Meditationes algebraicae (1770), which also contained Waring’s problem and what was later known as Vinogradov’s theorem. The latter, which states that every sufficiently large odd integer can be expressed as the sum of three primes, was proved in 1937 by the Russian mathematician Ivan Matveyevich Vinogradov. Further progress on Goldbach’s conjecture occurred in 1973, when the Chinese mathematician Chen Jing Run proved that every sufficiently large even number is the sum of a prime and a number with at most two prime factors.

**Details**

Goldbach's conjecture is one of the oldest and best-known unsolved problems in number theory and all of mathematics. It states that every even whole number greater than 2 is the sum of two prime numbers.

The conjecture has been shown to hold for all integers less than 4 × {10}^{18}, but remains unproven despite considerable effort.

**History**

On 7 June 1742, the German mathematician Christian Goldbach wrote a letter to Leonhard Euler (letter XLIII), in which he proposed the following conjecture:

*Every integer that can be written as the sum of two primes can also be written as the sum of as many primes as one wishes, until all terms are units.*

Goldbach was following the now-abandoned convention of considering 1 to be a prime number, so that a sum of units would indeed be a sum of primes. He then proposed a second conjecture in the margin of his letter, which implies the first:

*Every integer greater than 2 can be written as the sum of three primes.*

Euler replied in a letter dated 30 June 1742 and reminded Goldbach of an earlier conversation they had had ("... so Ew vormals mit mir communicirt haben ..."), in which Goldbach had remarked that the first of those two conjectures would follow from the statement

*Every positive even integer can be written as the sum of two primes.*

This is in fact equivalent to his second, marginal conjecture. In the letter dated 30 June 1742, Euler stated:

Dass ... ein jeder numerus par eine summa duorum primorum sey, halte ich für ein ganz gewisses theorema, ungeachtet ich dasselbe nicht demonstriren kann.

That ... every even integer is a sum of two primes, I regard as a completely certain theorem, although I cannot prove it.

Each of the three conjectures above has a natural analog in terms of the modern definition of a prime, under which 1 is excluded. A modern version of the first conjecture is:

*Every integer that can be written as the sum of two primes can also be written as the sum of as many primes as one wishes, until either all terms are two (if the integer is even) or one term is three and all other terms are two (if the integer is odd).*

A modern version of the marginal conjecture is:

*Every integer greater than 5 can be written as the sum of three primes.*

And a modern version of Goldbach's older conjecture of which Euler reminded him is:

*Every even integer greater than 2 can be written as the sum of two primes.*

These modern versions might not be entirely equivalent to the corresponding original statements. For example, if there were an even integer

larger than 4, for p a prime, that could not be expressed as the sum of two primes in the modern sense, then it would be a counterexample to the modern version of the third conjecture (without being a counterexample to the original version). The modern version is thus probably stronger (but in order to confirm that, one would have to prove that the first version, freely applied to any positive even integer n, could not possibly rule out the existence of such a specific counterexample N). In any case, the modern statements have the same relationships with each other as the older statements did. That is, the second and third modern statements are equivalent, and either implies the first modern statement.The third modern statement (equivalent to the second) is the form in which the conjecture is usually expressed today. It is also known as the "strong", "even", or "binary" Goldbach conjecture. A weaker form of the second modern statement, known as "Goldbach's weak conjecture", the "odd Goldbach conjecture", or the "ternary Goldbach conjecture," asserts that

*Every odd integer greater than 7 can be written as the sum of three odd primes.*

A proof for the weak conjecture was proposed in 2013 by Harald Helfgott. Helfgott's proof has not yet appeared in a peer-reviewed publication, though was accepted for publication in the Annals of Mathematics Studies series in 2015, and has been undergoing further review and revision since. The weak conjecture would be a corollary of the strong conjecture: if n – 3 is a sum of two primes, then n is a sum of three primes. However, the converse implication and thus the strong Goldbach conjecture remain unproven.

**Christian Goldbach**

Christian Goldbach, (born March 18, 1690, Königsberg, Prussia [now Kaliningrad, Russia]—died Nov. 20, 1764, Moscow, Russia), was a Russian mathematician whose contributions to number theory include Goldbach’s conjecture.

In 1725 Goldbach became professor of mathematics and historian of the Imperial Academy at St. Petersburg. Three years later he went to Moscow as tutor to Tsar Peter II, and from 1742 he served as a staff member of the Russian Ministry of Foreign Affairs.

Goldbach first proposed the conjecture that bears his name in a letter to the Swiss mathematician Leonhard Euler in 1742. He claimed that “every number greater than 2 is an aggregate of three prime numbers.” Because mathematicians in Goldbach’s day considered 1 a prime number (prime numbers are now defined as those positive integers greater than 1 that are divisible only by 1 and themselves), Goldbach’s conjecture is usually restated in modern terms as: Every even natural number greater than 2 is equal to the sum of two prime numbers.

The first breakthrough in the effort to prove Goldbach’s conjecture occurred in 1930, when the Soviet mathematician Lev Genrikhovich Shnirelman proved that every natural number can be expressed as the sum of not more than 20 prime numbers. In 1937 the Soviet mathematician Ivan Matveyevich Vinogradov went on to prove that every “sufficiently large” (without stating exactly how large) odd natural number can be expressed as the sum of not more than three prime numbers. The latest refinement came in 1973, when the Chinese mathematician Chen Jing Run proved that every sufficiently large even natural number is the sum of a prime and a product of at most two primes.

Goldbach also made notable contributions to the theory of curves, to infinite series, and to the integration of differential equations.

]]>