Binary Goldbach conjecture: is every even integer greater than 2 a sum of two primes?

The strong (binary) Goldbach conjecture, originating in the 1742 correspondence between Christian Goldbach and Leonhard Euler, asserts that every even integer greater than 2 can be written as the sum of two primes. Despite substantial partial results — Helfgott's proof of the ternary Goldbach conjecture, Chen's theorem on primes plus almost-primes, density-zero bounds on the exceptional set, and computational verification up to 4×10^18 — the conjecture in its full unrestricted form remains open. Recent work, such as a 2024 paper on density versions of the problem, addresses only almost-all and density-restricted variants.

Background

In a 1742 letter to Leonhard Euler, Christian Goldbach proposed a statement about representing integers as sums of primes; the form that has become canonical, confirmed by Euler as a natural restatement, is that every even integer greater than 2 is the sum of two primes (equivalently, every even integer $n\ge 4$ is $p+q$ with $p,q$ prime, where equal primes are allowed, e.g. $4=2+2$). This is called the strong or binary Goldbach conjecture; it implies the weak (ternary) conjecture that every odd integer greater than 5 is the sum of three primes.

The central tool for such additive problems is the circle method (Hardy and Littlewood's technique of expressing the number of representations as an integral over the unit circle of a generating function, then estimating contributions from major arcs near rational points with small denominator and minor arcs elsewhere). Vinogradov used it in 1937 to prove the ternary statement for all sufficiently large odd numbers, and Harald Helfgott completed the ternary conjecture in 2013 by strengthening both the major-arc and minor-arc estimates and verifying the remaining finite range computationally. The circle method does not directly settle the binary problem: for a two-prime sum the major arcs dominate the analysis in a way that requires distribution information about primes that current techniques cannot supply.

The best approximations to the binary conjecture are of three kinds. First, Chen's theorem (Chen Jing-run, 1973) states that every sufficiently large even integer is the sum of a prime and a $P_2$-number, i.e. an integer with at most two prime factors (counted with multiplicity); this remains the strongest result of the prime-plus-almost-prime type. Second, results on the exceptional set — the set of even integers up to $x$ that are not a sum of two primes — show it is small: Montgomery and Vaughan (1975) proved it has size bounded by $O(x^{1-\delta})$ for some $\delta>0$, so that the conjecture holds for almost all even integers (all except a set of natural density zero); subsequent work has shrunk the exponent but never shown the exceptional set is empty or bounded. Third, the conjecture has been verified by computation for all even integers up to $4\cdot 10^{18}$ (Oliveira e Silva, Herzog, Pardi, 2014). A common heuristic explanation for the difficulty is the parity barrier of sieve theory: standard sieve methods cannot distinguish numbers with an odd number of prime factors from those with an even number, and so cannot by themselves certify a number with exactly one prime factor.

A 2024 paper of Alsetri and Shao studies density versions of the binary Goldbach problem: if $A$ is a subset of the primes whose relative density in every reduced residue class (a congruence class $a \bmod m$ with $\gcd(a,m)=1$) is at least $\delta>1/2$, then almost all even integers are sums of two elements of $A$; the threshold $1/2$ is best possible. That work, like the almost-all results before it, leaves the original conjecture untouched, and its authors explicitly frame it against the still-unresolved classical problem.

Problem Statement

Prove or disprove the binary (strong) Goldbach conjecture: every even integer $n$ with $n\ge 4$ (equivalently, every even integer greater than 2) can be written as $n=p+q$ where $p$ and $q$ are both prime (the two primes may be equal, as in $4=2+2$). A resolution must cover all even integers without exception — partial results such as validity for almost all even integers, validity for sufficiently large even integers with a prime plus an almost-prime, or verification up to any finite bound do not settle the question.

References

1. Ali Alsetri and Xuancheng Shao, "Density versions of the binary Goldbach problem", Acta Arithmetica 218 (2025), 285–295, DOI 10.4064/aa240615-19-9; preprint arXiv:2405.18576 (2024), https://arxiv.org/abs/2405.18576
2. H. A. Helfgott, "Major arcs for Goldbach's problem" (2013), arXiv:1305.2897, https://arxiv.org/abs/1305.2897
3. H. A. Helfgott, "Minor arcs for Goldbach's problem" (2012), arXiv:1205.5252, https://arxiv.org/abs/1205.5252
4. T. Oliveira e Silva, S. Herzog, and S. Pardi, "Empirical verification of the even Goldbach conjecture and computation of prime gaps up to 4⋅10¹⁸", Mathematics of Computation 83 (2014), 2033–2060, DOI 10.1090/S0025-5718-2013-02787-1, https://doi.org/10.1090/S0025-5718-2013-02787-1
5. Jing-run Chen, "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 (1973), 157–176; reprint in The Goldbach Conjecture (World Scientific, Series in Pure Mathematics, 2002), pp. 275–294, DOI 10.1142/9789812776600_0021, https://doi.org/10.1142/9789812776600_0021
6. H. L. Montgomery and R. C. Vaughan, "The exceptional set of Goldbach's problem", Acta Arithmetica 27 (1975), 353–370, DOI 10.4064/aa-27-1-353-370, https://doi.org/10.4064/aa-27-1-353-370