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Goldbach's conjecture
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{{short description|Conjecture that every even integer greater than 2 is the sum of two primes}}
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- Goldbach partitions of the even integers from 4 to 50 rev4b.svg|thumb=Goldbach_partitions_of_the_even_integers_from_4_to_28_300px.png -
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.
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 can be expressed as the sum of two primes.{{MathWorld|title=Goldbach Conjecture|urlname=GoldbachConjecture}}
The conjecture has been shown to hold for all integers less than 4 Ã— 1018,WEB,weblink Goldbach conjecture verification, TomÃ¡s Oliveira e, Silva, www.ieeta.pt, but remains unproven despite considerable effort.

## Goldbach number

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- GoldbachConjecture.gif -
The number of ways an even number can be represented as the sum of two primes."Goldbach's Conjecture" by Hector Zenil, Wolfram Demonstrations Project, 2007.
A Goldbach number is a positive even integer that can be expressed as the sum of two odd primes.{{MathWorld|title=Goldbach Number|urlname=GoldbachNumber}} Since four is the only even number greater than two 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, ... {{OEIS|id=A045917}}.

## Origins

(File:Letter Goldbach-Euler.jpg|thumb|200px|Letter from Goldbach to Euler dated on 7. June 1742 (Latin-German).Correspondance mathÃ©matique et physique de quelques cÃ©lÃ¨bres gÃ©omÃ¨tres du XVIIIÃ¨me siÃ¨cle (Band 1), St.-PÃ©tersbourg 1843, pp. 125â€“129)On 7 June 1742, the German mathematician Christian Goldbach wrote a letter to Leonhard Euler (letter XLIIIweblink in which he proposed the following conjecture:
Every integer which 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.
He then proposed a second conjecture in the margin of his letter:
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. The two conjectures are now known to be equivalent, but this did not seem to be an issue at the time.A modern version of Goldbach's marginal conjecture is:
Every integer greater than 5 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(), in which Goldbachremarked his original (and not marginal) conjecture followed from the following statement
Every even integer greater than 2 can be written as the sum of two primes,
which is, thus, also a conjecture of Goldbach.In the letter dated 30 June 1742, Euler stated:
("That â€¦ every even integer is a sum of two primes, I regard as a completely certain theorem, although I cannot prove it.")WEB
, Ingham
, AE
, Popular Lectures
,
, yes
, 2003-06-16
, 2009-09-23
, WEB
, Caldwell
, Chris
, Goldbach's conjecture
, 2008
, 2008-08-13,
Goldbach's third version (equivalent to the two other versions) is the form in which the conjecture is usually expressed today. It is also known as the "strong", "even", or "binary" Goldbach conjecture, to distinguish it from a weaker conjecture, known today variously as the Goldbach's weak conjecture, the "odd" Goldbach conjecture, or the "ternary" Goldbach conjecture. This weak conjecture asserts that all odd numbers greater than 7 are the sum of three odd primes, and appears to have been proved in 2013.ARXIV, 1305.2897, Major arcs for Goldbach's theorem, Helfgott, H.A., math.NT, 2013, ARXIV, 1205.5252, Minor arcs for Goldbach's problem, Helfgott, H.A., math.NT, 2012, The weak conjecture is a corollary of the strong conjecture, as, if {{math|n â€“ 3}} is a sum of two primes, then {{math|n}} is a sum of three primes. The converse implication, and 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 â‰¤ 105.Pipping, Nils (1890-1982), "Die Goldbachsche Vermutung und der Goldbach-Vinogradovsche Satz." Acta Acad. Aboensis, Math. Phys. 11, 4â€“25, 1938. With the advent of computers, many more values of n have been checked; T. Oliveira e Silva is running a distributed computer search that has verified the conjecture for n â‰¤ 4 Ã— 1018 (and double-checked up to 4 Ã— 1017) as of 2013. One record from this search is that 3,325,581,707,333,960,528 is the smallest number that has no Goldbach partition with a prime below 9781.TomÃ¡s Oliveira e Silva, Goldbach conjecture verification. Retrieved 20 July 2013

## 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.
missing image!
- Goldbach-1000.svg|thumbnail|right|288px|Number of ways to write an even number n as the sum of two primes (4 â‰¤ n â‰¤ 1,000), {{OEIS|A002375}})Goldbach-1000000.png -
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 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 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
sum_{m=3}^{n/2} frac{1}{ln m} {1 over 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 Hardyâ€“Littlewood prime tuple conjecture) that for any fixed c â‰¥ 2, the number of representations of a large integer n as the sumof c primes n=p_1+cdots +p_c with p_1 leq cdots leq p_c should be asymptotically equal to
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 gamma_{c,p}(n) is the number of solutions to the equationn = q_1 + cdots + q_c mod p in modular arithmetic, subject to the constraints q_1,ldots,q_c neq 0 mod p. This formula has been rigorously proven to be asymptotically valid for c â‰¥ 3 from the work of Vinogradov, but is still only a conjecture when c=2.{{Citation needed|date=January 2016}} In the latter case, the above formula simplifies to 0 when n is odd, and to
2 Pi_2 left(prod_{pmid n; p geq 3} frac{p-1}{p-2}right) int_2^n frac{dx}{(ln x)^2}
approx 2 Pi_2 left(prod_{pmid n; p geq 3} frac{p-1}{p-2}right) frac{n}{(ln n)^2}when n is even, where Pi_2 is Hardyâ€“Littlewood's twin prime constant
Pi_2 := prod_{p geq 3} left(1 - frac{1}{(p-1)^2}right) = 0.6601618158ldots.
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.JOURNAL, Fliegel, Henry F., Robertson, Douglas S., 1989, Goldbach's Comet: the numbers related to Goldbach's Conjecture, Journal of Recreational Mathematics, 21, 1, 1â€“7,

## 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.OEIS, A066352, Pillai sequence, One can consider similar problems in which primes are replaced by other particular sets of numbers, such as the squares.
• It was proven by Lagrange that every positive integer is the sum of four squares. See Waring's problem and the related Waringâ€“Goldbach problem on sums of powers of primes.
• Hardy and Littlewood listed as their Conjecture I: "Every large odd number (n > 5) is the sum of a prime and the double of a prime." (Mathematics Magazine, 66.1 (1993): 45â€“47.) This conjecture is known as Lemoine's conjecture (also called Levy's conjecture).
• The Goldbach conjecture for practical numbers, a prime-like sequence of integers, was stated by Margenstern in 1984,JOURNAL, M., Margenstern, Results and conjectures about practical numbers, Comptes Rendus de l'AcadÃ©mie des Sciences, 299, 1984, 895â€“898, and proved by Melfi in 1996:JOURNAL, G., Melfi, On two conjectures about practical numbers, Journal of Number Theory, 56, 1996, 205â€“210, 10.1006/jnth.1996.0012, every even number is a sum of two practical numbers.

## References

{{Reflist|30em}}

{{Prime number conjectures}}

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