Sexy prime

In number theory, sexy primes are prime numbers that differ from each other by 6. For example, the numbers 5 and 11 are both sexy primes, because both are prime and 11 − 5 = 6.

The term "sexy prime" is a pun stemming from the Latin word for six: sex.

If p + 2 or p + 4 (where p is the lower prime) is also prime, then the sexy prime is part of a prime triplet. In August 2014 the Polymath group, seeking the proof of the twin prime conjecture, showed that if the generalized Elliott–Halberstam conjecture is proven, one can show the existence of infinitely many pairs of consecutive primes that differ by at most 6 and as such they are either twin, cousin or sexy primes.[1]

Primorial n# notation
Further information: Primorial § Definition for natural numbers

As used in this article, n# stands for the product 2 · 3 · 5 · 7 · … of all the primes ≤ n.

Types of groupings

Sexy prime pairs

The sexy primes (sequences OEIS: A023201 and OEIS: A046117 in OEIS) below 500 are:

(5,11), (7,13), (11,17), (13,19), (17,23), (23,29), (31,37), (37,43), (41,47), (47,53), (53,59), (61,67), (67,73), (73,79), (83,89), (97,103), (101,107), (103,109), (107,113), (131,137), (151,157), (157,163), (167,173), (173,179), (191,197), (193,199), (223,229), (227,233), (233,239), (251,257), (257,263), (263,269), (271,277), (277,283), (307,313), (311,317), (331,337), (347,353), (353,359), (367,373), (373,379), (383,389), (433,439), (443,449), (457,463), (461,467).

As of April 2022, the largest-known pair of sexy primes was found by S. Batalov and has 51,934 digits. The primes are:

p = 11922002779 x (2172486 - 286243) + 286245 - 5
p+6 = 11922002779 x (2172486 - 286243) + 286245 + 1[2]

Sexy prime triplets

Sexy primes can be extended to larger constellations. Triplets of primes (p, p+6, p+12) such that p+18 is composite are called sexy prime triplets. Those below 1,000 are (OEIS: A046118, OEIS: A046119, OEIS: A046120):

(7,13,19), (17,23,29), (31,37,43), (47,53,59), (67,73,79), (97,103,109), (101,107,113), (151,157,163), (167,173,179), (227,233,239), (257,263,269), (271,277,283), (347,353,359), (367,373,379), (557,563,569), (587,593,599), (607,613,619), (647,653,659), (727,733,739), (941,947,953), (971,977,983).

In January 2005 Ken Davis set a record for the largest-known sexy prime triplet with 5132 digits:

p = (84055657369 · 205881 · 4001# · (205881 · 4001# + 1) + 210) · (205881 · 4001# - 1) / 35 + 1. [3]

In May 2019, Peter Kaiser improved this record to 6,031 digits:

p = 10409207693×220000−1.[4]

Gerd Lamprecht improved the record to 6,116 digits in August 2019:

p = 20730011943×14221#+344231.[5]

Ken Davis further improved the record with a 6,180 digit Brillhart-Lehmer-Selfridge provable triplet in October 2019:

p = (72865897*809857*4801#*(809857*4801#+1)+210)*(809857*4801#-1)/35+1[6]

Norman Luhn & Gerd Lamprecht improved the record to 6,701 digits in October 2019:

p = 22582235875×222224+1.[7]

Serge Batalov improved the record to 15,004 digits in April 2022:

p = 2494779036241x249800+1.[8]

Sexy prime quadruplets

Sexy prime quadruplets (p, p+6, p+12, p+18) can only begin with primes ending in a 1 in their decimal representation (except for the quadruplet with p = 5). The sexy prime quadruplets below 1000 are (OEIS: A023271, OEIS: A046122, OEIS: A046123, OEIS: A046124):

(5,11,17,23), (11,17,23,29), (41,47,53,59), (61,67,73,79), (251,257,263,269), (601,607,613,619), (641,647,653,659).

In November 2005 the largest-known sexy prime quadruplet, found by Jens Kruse Andersen had 1,002 digits:

p = 411784973 · 2347# + 3301.[9]

In September 2010 Ken Davis announced a 1,004 digit quadruplet with p = 23333 + 1582534968299.[10]

In May 2019 Marek Hubal announced a 1,138 digit quadruplet with p = 1567237911 × 2677# + 3301.[11][12]

In June 2019 Peter Kaiser announced a 1,534 digit quadruplet with p = 19299420002127 × 25050 + 17233.[13]

In October 2019 Gerd Lamprecht and Norman Luhn announced a 3,025 digit quadruplet with p = 121152729080 × 7019#/1729 + 1.[14]

In July 2023 Ken Davis announced a 3,207 digit quadruplet with p = (1021328211729*2521#*(483*2521#+1)+2310)*(483*2521#-1)/210+1.[15]

Sexy prime quintuplets

In an arithmetic progression of five terms with common difference 6, one of the terms must be divisible by 5, because 5 and 6 are relatively prime. However, all multiples of 5 (except itself) cannot be prime numbers. Thus, the only sexy prime quintuplet is (5,11,17,23,29); no longer sequence of sexy primes is possible, since adding 6 to the last number in the set of sexy prime quintuplets (29) equals 35, which is a composite number.

See also

References
  1. D.H.J. Polymath (2014). "Variants of the Selberg sieve, and bounded intervals containing many primes". Research in the Mathematical Sciences. 1 (12). arXiv:1407.4897. doi:10.1186/s40687-014-0012-7. MR 3373710. S2CID 119699189.
  2. Batalov, S. "Let's find some large sexy prime pair[s]". mersenneforum.org. Retrieved 3 October 2019.
  3. Davis, Ken (January 2023). "sexy prime triplet". Prime Pages. Retrieved 24 January 2023.
  4. Kaiser, Peter (May 2019). "sexy prime triplet". Mersenne forum. Retrieved 13 May 2019.
  5. Andersen, Jens Kruse. "The largest known CPAP's". primerecords.dk. Retrieved 19 August 2019.
  6. Davis, Ken. "Brillhart-Lehmer-Selfridge provable triplet Oct 2019". primenumbers yahoo group. Retrieved 2 October 2019.
  7. Andersen, Jens Kruse. "The largest known CPAP's". primerecords.dk. Retrieved 13 October 2019.
  8. Batalov, Serge. "Consecutive primes arithmetic progression (d=6), Apr 2022". Primes.utm.edu.
  9. Andersen, Jens K. (November 2005). "Gigantic sexy and cousin primes". primenumbers yahoo group. Retrieved 27 January 2009.
  10. Davis, Ken (September 2010). "1004 sexy prime quadruplet". primenumbers yahoo group. Retrieved 2 September 2010.
  11. Hubal, Marek (May 2019). "CPAP's sexy prime". primenumbers yahoo group. Retrieved 10 May 2019.
  12. Andersen, Jens Kruse (May 2019). "Re: CPAP's sexy prime". primenumbers yahoo group. Retrieved 19 September 2019.
  13. Kaiser, Peter (June 2019). "Let's find some large sexy prime pair (and, perhaps, a triplet)". Mersenne forum. Retrieved 18 August 2019.
  14. Lamprecht, Gerd; Luhn, Norman (October 2019). "CPAP's sexy prime". primenumbers yahoo group. Retrieved 13 October 2019.
  15. "PrimePage Primes: (1021328211729 · 2521# · (483 · 2521# + 1) + 2310) · (483 · 2521# - 1)/210 + 1".
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