# Goldbach's conjecture

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Field Letter from Goldbach to Euler dated on 7 June 1742 (Latin-German)[1] Number theory Christian Goldbach 1742 Yes Goldbach's weak conjecture

Goldbach's conjecture is one of the oldest and best-known unsolved problems in number theory and all of mathematics. It states:

Every even integer greater than 2 is the sum of two primes.[2]

The conjecture has been shown to hold for all integers less than 4 × 1018,[3] but remains unproven despite considerable effort.

## Goldbach number

The even integers from 4 to 28 as sums of two primes: Even integers correspond to horizontal lines. For each prime, there are two oblique lines, one red and one blue. The sums of two primes are the intersections of one red and one blue line, marked by a circle. Thus the circles on a given horizontal line give all partitions of the corresponding even integer into the sum of two primes.
The number of ways an even number can be represented as the sum of two primes[4]

A Goldbach number is a positive even integer that can be expressed as the sum of two odd primes.[5] Since 4 is the only even number greater than 2 that requires the even prime 2 in order to be written as the sum of two primes, another form of the statement of Goldbach's conjecture is that all even integers greater than 4 are Goldbach numbers.

The expression of a given even number as a sum of two primes is called a Goldbach partition of that number. The following are examples of Goldbach partitions for some even numbers:

6 = 3 + 3
8 = 3 + 5
10 = 3 + 7 = 5 + 5
12 = 7 + 5
...
100 = 3 + 97 = 11 + 89 = 17 + 83 = 29 + 71 = 41 + 59 = 47 + 53
...

The number of ways in which 2n can be written as the sum of two primes (for n starting at 1) is:

0, 1, 1, 1, 2, 1, 2, 2, 2, 2, 3, 3, 3, 2, 3, 2, 4, 4, 2, 3, 4, 3, 4, 5, 4, 3, 5, 3, 4, 6, 3, 5, 6, 2, 5, 6, 5, 5, 7, 4, 5, 8, 5, 4, 9, 4, 5, 7, 3, 6, 8, 5, 6, 8, 6, 7, 10, 6, 6, 12, 4, 5, 10, 3, ... (sequence A045917 in the OEIS).

## Origins

On 7 June 1742, the German mathematician Christian Goldbach wrote a letter to Leonhard Euler (letter XLIII),[6] 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,[2] 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 is easily seen to imply the first:

Every integer greater than 2 can be written as the sum of three primes.[7]

Euler replied in a letter dated 30 June 1742[8] 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:[9][10]

"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 ${\displaystyle N=p+1}$ larger than 4, for ${\displaystyle p}$ 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 (without of course being a counterexample to the original version) of the third conjecture. 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 ${\displaystyle n}$, could not possibly rule out the existence of such a specific counterexample ${\displaystyle N}$). 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; it has however not yet appeared in a peer-reviewed publication.[11][12] Note that 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. But the converse implication and thus the strong Goldbach conjecture remain unproven.

## Verified results

For small values of n, the strong Goldbach conjecture (and hence the weak Goldbach conjecture) can be verified directly. For instance, Nils Pipping in 1938 laboriously verified the conjecture up to n ≤ 105.[13] With the advent of computers, many more values of n have been checked; T. Oliveira e Silva ran a distributed computer search that has verified the conjecture for n ≤ 4 × 1018 (and double-checked up to 4 × 1017) as of 2013. One record from this search is that 3325581707333960528 is the smallest number that has no Goldbach partition with a prime below 9781.[14]

## Heuristic justification

Statistical considerations that focus on the probabilistic distribution of prime numbers present informal evidence in favour of the conjecture (in both the weak and strong forms) for sufficiently large integers: the greater the integer, 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.

Number of ways to write an even number n as the sum of two primes (4 ≤ n ≤ 1,000), (sequence A002375 in the OEIS)
Number of ways to write an even number n as the sum of two primes (4 ≤ n ≤ 1000000)

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 m selected at random has roughly a ${\displaystyle 1/\ln m}$ chance of being prime. Thus if n is a large even integer and m is a number between 3 and n/2, then one might expect the probability of m and n − m simultaneously being prime to be ${\displaystyle 1{\big /}{\big [}\ln m\,\ln(n-m){\big ]}}$. If one pursues this heuristic, one might expect the total number of ways to write a large even integer n as the sum of two odd primes to be roughly

${\displaystyle \sum _{m=3}^{n/2}{\frac {1}{\ln m}}{\frac {1}{\ln(n-m)}}\approx {\frac {n}{2(\ln n)^{2}}}.}$

Since this quantity goes to infinity as n 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.

This heuristic argument is actually somewhat inaccurate, because it assumes that the events of m and n − m being prime are statistically independent of each other. For instance, if m is odd, then n − m is also odd, and if m is even, then n − m is even, a non-trivial relation because, besides the number 2, only odd numbers can be prime. Similarly, if n is divisible by 3, and m was already a prime distinct from 3, then n − m 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 c ≥ 2, the number of representations of a large integer n as the sum of c primes ${\displaystyle n=p_{1}+\cdots +p_{c}}$ with ${\displaystyle p_{1}\leq \cdots \leq p_{c}}$ should be asymptotically equal to

${\displaystyle \left(\prod _{p}{\frac {p\gamma _{c,p}(n)}{(p-1)^{c}}}\right)\int _{2\leq x_{1}\leq \cdots \leq x_{c}:x_{1}+\cdots +x_{c}=n}{\frac {dx_{1}\cdots dx_{c-1}}{\ln x_{1}\cdots \ln x_{c}}},}$

where the product is over all primes p, and ${\displaystyle \gamma _{c,p}(n)}$ is the number of solutions to the equation ${\displaystyle n=q_{1}+\cdots +q_{c}\mod p}$ in modular arithmetic, subject to the constraints ${\displaystyle q_{1},\ldots ,q_{c}\neq 0\mod p}$. This formula has been rigorously proven to be asymptotically valid for c ≥ 3 from the work of Vinogradov, but is still only a conjecture when ${\displaystyle c=2}$.[citation needed] In the latter case, the above formula simplifies to 0 when n is odd, and to

${\displaystyle 2\Pi _{2}\left(\prod _{p\mid n;p\geq 3}{\frac {p-1}{p-2}}\right)\int _{2}^{n}{\frac {dx}{(\ln x)^{2}}}\approx 2\Pi _{2}\left(\prod _{p\mid n;p\geq 3}{\frac {p-1}{p-2}}\right){\frac {n}{(\ln n)^{2}}}}$

when n is even, where ${\displaystyle \Pi _{2}}$ is Hardy–Littlewood's twin prime constant

${\displaystyle \Pi _{2}:=\prod _{p\geq 3}\left(1-{\frac {1}{(p-1)^{2}}}\right)=0.6601618158\ldots .}$

This 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.

The Goldbach partition functions shown here can be displayed as histograms, which informatively illustrate the above equations. See Goldbach's comet.[15]

## Rigorous results

The strong Goldbach conjecture is much more difficult than the weak Goldbach conjecture. Using Vinogradov's method, Chudakov,[16] Van der Corput,[17] and Estermann[18] showed that almost all even numbers can be written as the sum of two primes (in the sense that the fraction of even numbers which can be so written tends towards 1). In 1930, Lev Schnirelmann proved[19][20] that any natural number greater than 1 can be written as the sum of not more than C prime numbers, where C is an effectively computable constant, see Schnirelmann density. Schnirelmann's constant is the lowest number C with this property. Schnirelmann himself obtained C < 800000. This result was subsequently enhanced by many authors, such as Olivier Ramaré, who in 1995 showed that every even number n ≥ 4 is in fact the sum of at most 6 primes. The best known result currently stems from the proof of the weak Goldbach conjecture by Harald Helfgott,[21] which directly implies that every even number n ≥ 4 is the sum of at most 4 primes.[22][23]

In 1924 Hardy and Littlewood showed under the assumption of the generalized Riemann hypothesis that the amount of even numbers up to X violating the Goldbach conjecture is much less than ${\displaystyle X^{(1/2)+c}}$ for small c.[24]

Chen Jingrun showed in 1973 using the methods of sieve theory that every sufficiently large even number can be written as the sum of either two primes, or a prime and a semiprime (the product of two primes).[25] See Chen's theorem for further information.

In 1975, Hugh Montgomery and Robert Charles Vaughan showed that "most" even numbers are expressible as the sum of two primes. More precisely, they showed that there exist positive constants c and C such that for all sufficiently large numbers N, every even number less than N is the sum of two primes, with at most ${\displaystyle CN^{1-c}}$ exceptions. In particular, the set of even integers that are not the sum of two primes has density zero.

In 1951, Linnik proved the existence of a constant K such that every sufficiently large even number is the sum of two primes and at most K powers of 2. Roger Heath-Brown and Jan-Christoph Schlage-Puchta in 2002 found that K = 13 works.[26]

As with many famous conjectures in mathematics, there are a number of purported proofs of the Goldbach conjecture, none of which are accepted by the mathematical community.

## Related problems

Although Goldbach's conjecture implies that every positive integer greater than one can be written as a sum of at most three primes, it is not always possible to find such a sum using a greedy algorithm that uses the largest possible prime at each step. The Pillai sequence tracks the numbers requiring the largest number of primes in their greedy representations.[27]

One can consider similar problems in which primes are replaced by other particular sets of numbers, such as the squares.

## In popular culture

Goldbach's Conjecture (Chinese: 哥德巴赫猜想) is the title of the biography of Chinese mathematician and number theorist Chen Jingrun, written by Xu Chi.

The conjecture is a central point in the plot of the 1992 novel Uncle Petros and Goldbach's Conjecture by Greek author Apostolos Doxiadis, in the short story "Sixty Million Trillion Combinations" by Isaac Asimov and also in the 2008 mystery novel No One You Know by Michelle Richmond.[30]

Goldbach's conjecture is part of the plot of the Spanish film Fermat's Room (es:La habitación de Fermat) (2007).

## References

1. ^ Correspondance mathématique et physique de quelques célèbres géomètres du XVIIIème siècle (Band 1), St.-Pétersbourg 1843, pp. 125–129.
2. ^ a b Weisstein, Eric W. "Goldbach Conjecture". MathWorld.
3. ^ Silva, Tomás Oliveira e. "Goldbach conjecture verification". www.ieeta.pt.
4. ^ "Goldbach's Conjecture" by Hector Zenil, Wolfram Demonstrations Project, 2007.
5. ^
6. ^ http://www.math.dartmouth.edu/~euler/correspondence/letters/OO0765.pdf
7. ^ In the printed version published by P.H. Fuss [1] 2 is misprinted as 1 in the marginal conjecture.
8. ^ http://eulerarchive.maa.org//correspondence/letters/OO0766.pdf
9. ^ Ingham, A. E. "Popular Lectures" (PDF). Archived from the original (PDF) on 2003-06-16. Retrieved 2009-09-23.
10. ^ Caldwell, Chris (2008). "Goldbach's conjecture". Retrieved 2008-08-13.
11. ^ Helfgott, H. A. (2013). "Major arcs for Goldbach's theorem". arXiv:1305.2897 [math.NT].
12. ^ Helfgott, H. A. (2012). "Minor arcs for Goldbach's problem". arXiv:1205.5252 [math.NT].
13. ^ Pipping, Nils (1890–1982), "Die Goldbachsche Vermutung und der Goldbach-Vinogradowsche Satz". Acta Acad. Aboensis, Math. Phys. 11, 4–25, 1938.
14. ^ Tomás Oliveira e Silva, Goldbach conjecture verification. Retrieved 20 July 2013.
15. ^ Fliegel, Henry F.; Robertson, Douglas S. (1989). "Goldbach's Comet: the numbers related to Goldbach's Conjecture". Journal of Recreational Mathematics. 21 (1): 1–7.
16. ^ Chudakov, Nikolai G. (1937). "О проблеме Гольдбаха" [On the Goldbach problem]. Doklady Akademii Nauk SSSR. 17: 335–338.
17. ^ Van der Corput, J. G. (1938). "Sur l'hypothèse de Goldbach" (PDF). Proc. Akad. Wet. Amsterdam (in French). 41: 76–80.
18. ^ Estermann, T. (1938). "On Goldbach's problem: proof that almost all even positive integers are sums of two primes". Proc. London Math. Soc. 2. 44: 307–314. doi:10.1112/plms/s2-44.4.307.
19. ^ Schnirelmann, L. G. (1930). "On the additive properties of numbers", first published in "Proceedings of the Don Polytechnic Institute in Novocherkassk" (in Russian), vol XIV (1930), pp. 3–27, and reprinted in "Uspekhi Matematicheskikh Nauk" (in Russian), 1939, no. 6, 9–25.
20. ^ Schnirelmann, L. G. (1933). First published as "Über additive Eigenschaften von Zahlen" in "Mathematische Annalen" (in German), vol. 107 (1933), 649–690, and reprinted as "On the additive properties of numbers" in "Uspekhi Matematicheskikh Nauk" (in Russian), 1940, no. 7, 7–46.
21. ^ Helfgott, H. A. (2013). "The ternary Goldbach conjecture is true". arXiv:1312.7748 [math.NT].
22. ^ Sinisalo, Matti K. (Oct 1993). "Checking the Goldbach Conjecture up to 4 1011" (PDF). Mathematics of Computation. American Mathematical Society. 61 (204): 931–934. CiteSeerX 10.1.1.364.3111. doi:10.2307/2153264. JSTOR 2153264.
23. ^ Rassias, M. Th. (2017). Goldbach's Problem: Selected Topics. Springer.
24. ^ See, for example, A new explicit formula in the additive theory of primes with applications I. The explicit formula for the Goldbach and Generalized Twin Prime Problems by Janos Pintz.
25. ^ Chen, J. R. (1973). "On the representation of a larger even integer as the sum of a prime and the product of at most two primes". Sci. Sinica. 16: 157–176.
26. ^ Heath-Brown, D. R.; Puchta, J. C. (2002). "Integers represented as a sum of primes and powers of two". Asian Journal of Mathematics. 6 (3): 535–565. arXiv:math.NT/0201299. doi:10.4310/AJM.2002.v6.n3.a7. S2CID 2843509.
27. ^ Sloane, N. J. A. (ed.). "Sequence A066352 (Pillai sequence)". The On-Line Encyclopedia of Integer Sequences. OEIS Foundation.
28. ^ Margenstern, M. (1984). "Results and conjectures about practical numbers". Comptes rendus de l'Académie des Sciences. 299: 895–898.
29. ^ Melfi, G. (1996). "On two conjectures about practical numbers". Journal of Number Theory. 56: 205–210. doi:10.1006/jnth.1996.0012.
30. ^ "MathFiction: No One You Know (Michelle Richmond)". kasmana.people.cofc.edu.