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This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.

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A076336 (Provable) Sierpiński numbers: odd numbers n such that for all k >= 1 the numbers n*2^k + 1 are composite.

Original entry on oeis.org

78557, 271129, 271577, 322523, 327739, 482719, 575041, 603713, 903983, 934909, 965431, 1259779, 1290677, 1518781, 1624097, 1639459, 1777613, 2131043, 2131099, 2191531, 2510177, 2541601, 2576089, 2931767, 2931991, 3083723, 3098059, 3555593, 3608251
Offset: 1

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Author

N. J. A. Sloane, Nov 07 2002

Keywords

Comments

"Provable" in the definition means provable by any method (whether using a covering set or not). - N. J. A. Sloane, Aug 03 2024
It is only a conjecture that this sequence is complete up to 3000000 - there may be missing terms.
It is conjectured that 78557 is the smallest Sierpiński number. - T. D. Noe, Oct 31 2003
Sierpiński numbers may be proved by exhibiting a periodic sequence p of prime divisors with p(k) | n*2^k+1 and disproved by finding a prime n*2^k+1. It is conjectured by some people that numbers that cannot be proved to be Sierpiński by this method are non-Sierpiński. However, some numbers resist both proof and disproof. - David W. Wilson, Jan 17 2005 [Edited by N. J. A. Sloane, Aug 03 2024]
Sierpiński showed that this sequence is infinite.
There are four related sequences that arise in this context:
S1: Numbers n such that n*2^k + 1 is composite for all k (this sequence)
S2: Odd numbers n such that 2^k + n is composite for all k (apparently it is conjectured that S1 and S2 are the same sequence)
S3: Numbers n such that n*2^k + 1 is prime for all k (empty)
S4: Numbers n such that 2^k + n is prime for all k (empty)
The following argument, due to Michael Reid, attempts to show that S3 and S4 are empty: If p is a prime divisor of n + 1, then for k = p - 1, the term (either n*2^k + 1 or 2^k + n) is a multiple of p (and also > p, so not prime). [However, David McAfferty points that for the case S3, this argument fails if p is of the form 2^m-1. So it may only be a conjecture that the set S3 is empty. - N. J. A. Sloane, Jun 27 2021]
a(1) = 78557 is also the smallest odd n for which either n^p*2^k + 1 or n^p + 2^k is composite for every k > 0 and every prime p greater than 3. - Arkadiusz Wesolowski, Oct 12 2015
n = 4008735125781478102999926000625 = (A213353(1))^4 is in this sequence but is thought not to satisfy the conjecture mentioned by David W. Wilson above. For this multiplier, all n*2^(4m + 2) + 1 are composite by an Aurifeuillean factorization. Only the remaining cases, n*2^k + 1 where k is not 2 modulo 4, are covered by a finite set of primes (namely {3, 17, 97, 241, 257, 673}). See Izotov link for details (although with another prime set). - Jeppe Stig Nielsen, Apr 14 2018
Conjecture: if S is a (provable) Sierpiński number, then there exists a prime P such that S^p is also a Sierpiński number for every prime p > P. - Thomas Ordowski, Jul 12 2022
Problem: are there odd numbers K such that K - 2^m is a Sierpiński number for every 1 < 2^m < K? If so, then all positive values of (K - 2^m)*2^n + 1 are composite. Also, by the dual Sierpiński conjecture, K - 2^m + 2^n is composite for every 1 < 2^m < K and for every n > 0. Note that, by the dual Sierpiński conjecture, if p is an odd prime and 1 < 2^m < p, then there exists n such that (p - 2^m)*2^n + 1 is prime. So if such a number K exists, it must be composite. - Thomas Ordowski, Jul 20 2022
From M. F. Hasler, Jul 23 2022: (Start)
1) The above Conjecture is true for Sierpiński numbers provable by a "covering set", with P equal to the largest prime factor of the elements of that set*, according to the explanation from Michael Filaseta posted Jul 12 2022 on the SeqFan mailing list, cf. links. (*More generally: for S^p with any p coprime to all elements of the covering set, but not necessarily prime.)
2) Wilson's comment from 2005 (also the first part, not only the conjecture) is misleading if not wrong because there are provable Sierpiński numbers for which a covering set is not known (maybe even believed not to exist), as explained by Nielsen in his above comment from 2018. (End)

References

  • R. K. Guy, Unsolved Problems in Number Theory, Section B21.
  • C. A. Pickover, The Math Book, Sterling, NY, 2009; see p. 420.
  • Paulo Ribenboim, The Book of Prime Number Records, 2nd. ed., 1989, p. 282.
  • Paulo Ribenboim, The Little Book of Bigger Primes, Springer-Verlag NY 2004. See pp. 237-238.

Links

Crossrefs

Cf. A003261, A052333, A076335, A076337, A101036, A137715, A263169, A305473, A306151.

A306151 Let k be a Sierpiński or Riesel number, and let p be the largest number in a set of n primes which cover every number of the form k*2^m + 1 (or of the form k*2^m - 1) with m >= 1. a(n) = 0 if no covering set with n primes exists, otherwise a(n) = p if and only if there exists no number k that has a covering set with precisely n primes and with largest prime < p.

Original entry on oeis.org

0, 0, 0, 0, 0, 241, 73, 241, 151, 241, 151, 151, 241, 257, 257, 257
Offset: 1

Views

Author

Arkadiusz Wesolowski, Jun 23 2018

Keywords

Examples

			Examples of the covering sets:
- for n = 6, the set is {3, 5, 7, 13, 17, 241},
- for n = 7, the set is {3, 5, 7, 13, 19, 37, 73},
- for n = 8, the set is {3, 5, 7, 17, 19, 37, 73, 241},
- for n = 9, the set is {3, 5, 7, 11, 13, 31, 41, 61, 151},
- for n = 10, the set is {3, 5, 7, 11, 17, 31, 41, 61, 151, 241},
- for n = 11, the set is {3, 5, 7, 11, 19, 31, 37, 41, 61, 73, 151},
- for n = 12, the set is {3, 7, 11, 13, 19, 31, 37, 41, 61, 73, 109, 151},
- for n = 13, the set is {3, 7, 11, 17, 19, 31, 37, 41, 61, 73, 109, 151, 241},
- for n = 14, the set is {3, 7, 11, 17, 19, 31, 37, 41, 61, 73, 97, 109, 151, 257},
- for n = 15, the set is {3, 11, 13, 17, 19, 31, 37, 41, 61, 73, 97, 109, 151, 241, 257},
- for n = 16, the set is {5, 7, 11, 13, 17, 19, 31, 37, 41, 61, 73, 97, 109, 151, 241, 257}.

Crossrefs

Cf. A076336, A101036, A206001, A206430, A305473.

Extensions

Corrected by Arkadiusz Wesolowski, Aug 04 2023
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