# Goldbach conjecture verification

## Introduction

The Goldbach conjecture is one of the oldest unsolved problems in number theory [1, problem C1]. In its modern form, it states that every even number larger than two can be expressed as a sum of two prime numbers.

Let n be an even number larger than two, and let n=p+q, with p and q prime numbers, p<=q, be a Goldbach partition of n. Let r(n) be the number of Goldbach partitions of n. The number of ways of writing n as a sum of two prime numbers, when the order of the two primes is important, is thus R(n)=2r(n) when n/2 is not a prime and is R(n)=2r(n)-1 when n/2 is a prime. The Goldbach conjecture states that r(n)>0, or, equivalently, that R(n)>0, for every even n larger than two.

In their famous memoir [2, conjecture A], Hardy and Littlewood conjectured that when n tends to infinity, R(n) tends asymptotically to (i.e., the ratio of the two functions tends to one)

```                         n                         p-1
N2(n) = 2 C       ----------------     PRODUCT     --- ,
twin   (log n)(log n-2)   p odd prime   p-2
divisor of n
```

where

```                       p(p-2)
C     =    PRODUCT     -------  = 0.66016181584686957392...
twin    p odd prime   (p-1)^2
```

is the twin primes constant. In [3], Crandall and Pomerance suggest replacing the factor

```       n
----------------
(log n)(log n-2)
```

appearing in the formula of N2(n) by the asymptotically equivalent factor

```        n-2         dx
INTEGRAL      --------------- .
2     log(x) log(n-x)
```

The numerical evidence supporting this conjectured asymptotic formula is very strong. Up to 10^10, the Crandall-Pomerance formula does not deviate from R(n) by more than 40150, and up to 2^40 it does not deviate from R(n) by more than 401900.

Let us order the r(n) Goldbach partitions of n by increasing order of the smallest prime of the partition. More precisely, let us denote the two primes of the i-th Goldbach partition of n by p(n;i) and q(n;i), with p(n;i) <= q(n;i) and p(n;i) < p(n;i+1). In order to verify the Goldbach conjecture for a given n, it is sufficient to find one of its Goldbach partitions. Our strategy will be to find the minimal Goldbach partition n=p(n;1)+q(n;1), i.e., the one that uses the smallest possible prime number p(n)=p(n;1). As in [4], for every prime q we will denote by S(q) the least even number n such that p(n)=q.

## News

February 1, 2005: 2·10^17 reached.
March 20, 2005: 10^17 double checked.
December 26, 2005: 3·10^17 reached.
June 5, 2006: 4·10^17 reached.
September 28, 2006: interval between 7·10^17 and 10^18 checked.
February 19, 2007: 5·10^17 reached.
February 23, 2007: interval between 6·10^17 and 7·10^17 checked.
April 25, 2007: 10^18 reached.
February 16, 2008: 11·10^17 reached.
July 14, 2008: 12·10^17 reached.
July 24, 2009: 15·10^17 reached.
December 23, 2009: 16·10^17 reached.
January 5, 2010: interval between 19·10^17 and 20·10^17 checked.
March 24, 2010: interval between 18·10^17 and 19·10^17 checked.
August 24, 2010: interval between 17·10^17 and 18·10^17 checked.
November 6, 2010: 2·10^18 reached.
May 24, 2011: 22·10^17 reached.
July 31, 2011: 25·10^17 reached.
September 13, 2011: 26·10^17 reached.
November 30, 2011: discovery of the minimal Goldbach partition 2795935116574469638=9629+P19.
January 16, 2012: discovery of the minimal Goldbach partition 3325581707333960528=9781+P19.
February 6, 2012: 34·10^17 reached.
February 13, 2012: 35·10^17 reached.
April 4, 2012: 4·10^18, our desired verification limit, reached.
June 18, 2012: 2·10^17 double checked.
September 7, 2012: 3·10^17 double checked.
May 26, 2013: 4·10^17 double checked. No more double checking will be done in the near future.

## Computational results

We have implemented a program that finds the minimal Goldbach partition of every even integer larger than four. In order to do this efficiently, the computation intensive parts of the program were written in assembly language (for the IA32 instruction set). A very efficient cache friendly implementation of the segmented sieve of Eratosthenes was used to generate the prime numbers (see our speed comparison chart [23KiB, PDF] between several Intel and AMD CPUs). For each interval of 10^12 integers, we record the number of times each (small) prime is used in a minimal Goldbach partition, as well as the even integer where it was first needed. Because it takes very little extra time, we also record information about the gaps between consecutive primes, viz., how many times each gap occurs, and its first occurrence. On a single core of a 3.3GHz core i3 processor, testing an interval of 10^12 integers near 10^18 takes close to 48 minutes. The execution time of the program grows very slowly, like log(N), where N is the last integer of the interval being tested, and it uses an amount of memory that is roughly given by 13 sqrt(N) / log(N). The program ran on the spare time of many computers, either under GNU/Linux or under Windows XP. We have reached 2·10^18 in November 2010, and in April 2012 have finally reached 4·10^18.

The following table presents an overview of the current status of this massive computation. Each cell represents an interval of 10^15; its background color indicates its computational status (green for double-checked, yellow for single-checked, and red for not yet done or not yet fully checked), and its brightness indicates if counts of the primes in each of the 32 residue classes modulo 120 are available (bright) or not (not so bright) to perform an initial check of the correctness of the computation on each interval of 10^15 (prime counts for each interval of 10^12 are also available to perform correctness checks).

 0000 0001 0002 0003 0004 0005 0006 0007 0008 0009 0010 0011 0012 0013 0014 0015 0016 0017 0018 0019 ... (same state) ... 0380 0381 0382 0383 0384 0385 0386 0387 0388 0389 0390 0391 0392 0393 0394 0395 0396 0397 0398 0399 0400 0401 0402 0403 0404 0405 0406 0407 0408 0409 0410 0411 0412 0413 0414 0415 0416 0417 0418 0419 ... (same state) ... 0960 0961 0962 0963 0964 0965 0966 0967 0968 0969 0970 0971 0972 0973 0974 0975 0976 0977 0978 0979 0980 0981 0982 0983 0984 0985 0986 0987 0988 0989 0990 0991 0992 0993 0994 0995 0996 0997 0998 0999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 ... (same state) ... 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 ... (same state) ... 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 ... (same state) ... 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 ... (same state) ... 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 1783 1784 1785 1786 1787 1788 1789 1790 1791 1792 1793 1794 1795 1796 1797 1798 1799 1800 1801 1802 1803 1804 1805 1806 1807 1808 1809 1810 1811 1812 1813 1814 1815 1816 1817 1818 1819 ... (same state) ... 1900 1901 1902 1903 1904 1905 1906 1907 1908 1909 1910 1911 1912 1913 1914 1915 1916 1917 1918 1919 1920 1921 1922 1923 1924 1925 1926 1927 1928 1929 1930 1931 1932 1933 1934 1935 1936 1937 1938 1939 1940 1941 1942 1943 1944 1945 1946 1947 1948 1949 1950 1951 1952 1953 1954 1955 1956 1957 1958 1959 1960 1961 1962 1963 1964 1965 1966 1967 1968 1969 1970 1971 1972 1973 1974 1975 1976 1977 1978 1979 1980 1981 1982 1983 1984 1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 2019 ... (same state) ... 2220 2221 2222 2223 2224 2225 2226 2227 2228 2229 2230 2231 2232 2233 2234 2235 2236 2237 2238 2239 2240 2241 2242 2243 2244 2245 2246 2247 2248 2249 2250 2251 2252 2253 2254 2255 2256 2257 2258 2259 2260 2261 2262 2263 2264 2265 2266 2267 2268 2269 2270 2271 2272 2273 2274 2275 2276 2277 2278 2279 ... (same state) ... 2960 2961 2962 2963 2964 2965 2966 2967 2968 2969 2970 2971 2972 2973 2974 2975 2976 2977 2978 2979 2980 2981 2982 2983 2984 2985 2986 2987 2988 2989 2990 2991 2992 2993 2994 2995 2996 2997 2998 2999 3000 3001 3002 3003 3004 3005 3006 3007 3008 3009 3010 3011 3012 3013 3014 3015 3016 3017 3018 3019 3020 3021 3022 3023 3024 3025 3026 3027 3028 3029 3030 3031 3032 3033 3034 3035 3036 3037 3038 3039 ... (same state) ... 3800 3801 3802 3803 3804 3805 3806 3807 3808 3809 3810 3811 3812 3813 3814 3815 3816 3817 3818 3819 3820 3821 3822 3823 3824 3825 3826 3827 3828 3829 3830 3831 3832 3833 3834 3835 3836 3837 3838 3839 3840 3841 3842 3843 3844 3845 3846 3847 3848 3849 3850 3851 3852 3853 3854 3855 3856 3857 3858 3859 ... (same state) ... 3980 3981 3982 3983 3984 3985 3986 3987 3988 3989 3990 3991 3992 3993 3994 3995 3996 3997 3998 3999

We have tested all consecutive even numbers up to 4·10^18, and, so far, double-checked our results up to 4·10^17 (in all, 581701 intervals of 10^12 were double-checked). About 781.8 single-core CPU years were used to do all this.

As far as we are aware, the previous record of computation was 4·10^14 [5]. As expected, no counter-example of the conjecture was found. In this table [24KiB, compressed with gzip] we present all values of S(p) we were able to compute, as well as counts of the number of times each (small) prime was used in a minimal Goldbach partition. The record-holders, i.e., numbers larger than all previous ones of the same kind, are clearly marked in the table, which extends the tables 1 and 2 of [4] and table 1 of [5]. The following figure presents a graph with the available values of S(p).

The values of S(p) are bounded, for our empirical data, by the functions

```                         0.4                                     0.4
0.4   p                                 0.4   p
S_min(p) = 0.06  p     e        and    S_max(p) = 11.05  p     e     .
```

The two kinds of record-holders marked in the table mentioned above correspond to the values of S(p) closer to one of the two bounds. For large p the values of S(p) appear to be slowly approaching the upper bound; hence, the asymptotic growth rate of S(p) probably has a different functional form. In [6] it was stated, based on probabilistic considerations, that p should not grow faster than log^2 S(p) log log S(p). Our data is not enough to resolve this conjecture; the black curve corresponds to the line 1.527 log^2 S(p) log log S(p) (according to the conjecture, all data points should be above the line). It was found that for all our data p can be reasonably well approximated by 0.33(log S(p) log log S(p))^2. In [7] other, probably better, approximations to S(p) are studied in more detail.

Let D(x;p) be the relative frequency of occurrence of the prime p in the minimal Goldbach partition of the even numbers not larger than x. The following figure presents a graph of this function, computed for our current verification limit of the Goldbach conjecture.

Besides the expected near exponential decay of D(x;p), it is interesting to observe that there exists a distinct difference of behavior in the values of this function when p is a multiple of three plus one (white dots) and when it is not (yellow dots).

### Hardy-Littlewood constants

If one assumes the truth of the prime k-tuple conjecture [2], it is possible to estimate the number of occurrences up to x of minimal Goldbach partitions with a smallest prime of p, denoted here by L(x;p), for relatively small values of p [7]. We managed to compute all relevant constants necessary to do this for all (odd primes) p smaller than 250. The constants, and minimal details about how they can be used to estimate L(x;p), can be found here [47KiB, compressed with gzip].

## Top 50

The following table presents the 50 largest p (least primes of a Goldbach partition) found so far, with S(p)<4·10^18, either by contributors to this project or by other discoverers (those have an * before the discoverer name). Repeated values of p were excluded from this list.

Rank     p     S(p)     Discoverer
1     9781     3325 58170 73339 60528     Silvio Pardi
2     9629     2795 93511 65744 69638     Silvio Pardi
3     9341     906 03057 95622 79642     John Fettig & Nahil Sobh
4     9203     1348 11357 94295 47486     Siegfried "Zig" Herzog
5     9161     887 12380 30778 37868     Siegfried "Zig" Herzog
6     9091     3164 06916 06618 44912     Silvio Pardi
7     9001     3893 00922 74334 20582     Tomás Oliveira e Silva
8     8971     2588 35699 18831 39892     Tomás Oliveira e Silva
9     8951     914 47723 42519 16254     John Fettig & Nahil Sobh
10     8941     555 27435 15567 50822     Siegfried "Zig" Herzog
11     8933     258 54942 69161 49682     Siegfried "Zig" Herzog
12     8929     3124 35916 66041 72278     Silvio Pardi
13     8831     2408 33984 92355 52478     Tomás Oliveira e Silva
14     8821     1670 95614 81435 30128     Tomás Oliveira e Silva
15     8819     2717 57304 70073 92768     Silvio Pardi
16     8779     2314 72006 67403 14852     Tomás Oliveira e Silva
17     8761     3239 05756 38344 11028     Silvio Pardi
18     8747     3732 28263 42720 65914     Tomás Oliveira e Silva
19     8737     764 63115 78502 42766     Siegfried "Zig" Herzog
20     8731     2390 68041 75101 89328     Tomás Oliveira e Silva
21     8719     1570 80604 90039 48202     Tomás Oliveira e Silva
22     8707     2171 73319 79842 32734     Siegfried "Zig" Herzog
23     8699     2994 28857 66127 17268     Tomás Oliveira e Silva
24     8693     2046 38924 98824 24466     Tomás Oliveira e Silva
25     8689     1302 37600 11943 70768     Siegfried "Zig" Herzog
26     8681     771 06523 23704 26528     Siegfried "Zig" Herzog
27     8677     1928 32489 01056 96568     Tomás Oliveira e Silva
28     8663     1262 36426 84331 28726     Siegfried "Zig" Herzog
29     8647     2725 05867 19612 97876     Silvio Pardi
30     8641     517 71184 25980 37624     Tomás Oliveira e Silva
31     8629     1238 28931 11321 16112     Siegfried "Zig" Herzog
32     8623     1211 65003 16991 77606     Tomás Oliveira e Silva
33     8609     1872 49629 32659 49398     Siegfried "Zig" Herzog
34     8599     3556 57986 77406 73142     Silvio Pardi
35     8597     2218 70802 03257 72974     Siegfried "Zig" Herzog
36     8581     3663 29859 59299 27532     Silvio Pardi
37     8573     1134 05983 29344 00206     Siegfried "Zig" Herzog
38     8563     280 46026 69116 44252     Tomás Oliveira e Silva
39     8543     2297 37834 24924 06154     Tomás Oliveira e Silva
40     8539     941 90839 15563 21548     Siegfried "Zig" Herzog
41     8527     1295 15748 05397 26954     Siegfried "Zig" Herzog
42     8521     1176 80059 43587 37918     Siegfried "Zig" Herzog
43     8513     3431 34334 19613 51016     Silvio Pardi
44     8501     255 32912 66885 55994     Siegfried "Zig" Herzog
45     8467     2576 43841 05051 31868     Tomás Oliveira e Silva
46     8461     2565 58105 68187 05782     Tomás Oliveira e Silva
47     8447     2764 13588 98014 80338     Silvio Pardi
48     8443     121 00502 23040 07026     Tomás Oliveira e Silva
49     8431     2290 65390 58512 00938     Tomás Oliveira e Silva
50     8419     1730 12004 22605 42848     Tomás Oliveira e Silva

## Contributors

The following table presents some details about the contribution of all (past and present) individuals or organizations which donated computing power to this project.

Name     Location     Number of
Number of first (known) occurrences
Minimal Goldbach partitions     Prime gaps
Tomás Oliveira e Silva     All     2100920     325 (0)     70 (0)
DETUA     1510546     191 (0)     51 (0)
Home     440316     11 (0)     12 (0)
Kraken     109000     4 (0)     1 (0)
IEETA     41058     119 (0)     6 (0)
Siegfried "Zig" Herzog     PSU     1471605     80 (0)     52 (0)
Silvio Pardi     INFN     869080     15 (0)     9 (0)
Christian Kern     Germany     48999     3 (0)     2 (0)
John Fettig & Nahil Sobh     NCSA     33641     2 (0)     1 (0)
João Manuel Rodrigues     DETUA     16072     9 (0)     2 (0)
António Teixeira     IEETA     14995     0 (0)     1 (0)
Carlos Bastos     DETUA     8646     11 (0)     1 (0)
SIAS Group     IEETA     7752     2 (0)     0 (0)
Rui Arnaldo Costa     IEETA     3847     4 (0)     0 (0)
Armando Pinho     IEETA     3285     2 (0)     1 (0)
Miguel Oliveira e Silva     DETUA     2659     7 (0)     0 (0)
Laurent Desnoguès     France     200     0 (0)     0 (0)
All     All     4581701     460 (0)     139 (0)

This work was partially supported by the National Center for Supercomputing Applications and utilized the NCSA Xeon cluster.

This research was partially supported by an allocation of advanced computing resources supported by the National Science Foundation. Part of the computations were performed on Kraken (a Cray XT5) at the National Institute for Computational Sciences.

## References

 [1] Richard K. Guy, Unsolved problems in number theory, third edition, Springer-Verlag, 2004. [2] G. H. Hardy and J. E. Littlewood, Some problems of `partitio numerorum'; III: on the expression of a number as a sum of primes, Acta Mathematica, vol. 44, pp. 1-70, 1922. [3] Richard Crandall and Carl Pomerance, Prime numbers: a computational perspective, Springer-Verlag, 2001. [4] Matti K. Sinisalo, Checking the Goldbach conjecture up to 4·10^11, Mathematics of Computation, vol. 61, no. 204, pp. 931-934, October 1993. [5] Jörg Richstein, Verifying the Goldbach conjecture up to 4·10^14, Mathematics of Computation, vol. 70, no. 236, pp. 1745-1749, July 2000. [6] A. Granville, J. van de Lune, and H. J. J. te Riele, Checking the Goldbach conjecture on a vector computer, in Number Theory and Applications, R. A. Mollin (ed.), pp. 423-433, Kluwer Academic Press, 1989. [7] Tomás Oliveira e Silva, Siegfried Herzog, and Silvio Pardi, Empirical verification of the even Goldbach conjecture and computation of prime gaps up to 4·10^18, Mathematics of Computation, vol. 83, no. 288, pp. 2033-2060, July 2014 (published electronically on November 18, 2013).