

A000978


Wagstaff numbers: numbers n such that (2^n + 1)/3 is prime.
(Formerly M2413 N0956)


76



3, 5, 7, 11, 13, 17, 19, 23, 31, 43, 61, 79, 101, 127, 167, 191, 199, 313, 347, 701, 1709, 2617, 3539, 5807, 10501, 10691, 11279, 12391, 14479, 42737, 83339, 95369, 117239, 127031, 138937, 141079, 267017, 269987, 374321, 986191, 4031399
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OFFSET

1,1


COMMENTS

It is easy to see that the definition implies that n must be an odd prime.  N. J. A. Sloane, Oct 06 2006
The terms from a(32) on only give probable primes as of 2018. Caldwell lists the largest certified primes.  Jens Kruse Andersen, Jan 10 2018
Prime numbers of the form 1+Sum_{i=1..m} 2^(2i1).  Artur Jasinski, Feb 09 2007
There is a new conjecture stating that a Wagstaff number is prime under the following condition (based on DiGraph cycles under the LLT): Let p be a prime integer > 3, N(p) = 2^p+1 and W(p) = N(p)/3, S(0) = 3/2 (or 1/4) and S(i+1) = S(i)^2  2 (mod N(p)). Then W(p) is prime iff S(p1) == S(0) (mod W(p)).  Tony Reix (tony.reix(AT)laposte.net), Sep 03 2007
As a member of the DUR team (Diepeveen, Underwood, Reix), and thanks to the LLR tool built by Jean Penne, I've found a new and big Wagstaff PRP: (2^4031399+1)/3 is VrbaReix PRP! This Wagstaff number has 1,213,572 digits and today is the 3rd biggest PRP ever found. I've done a second verification on a Nehalem core with the PFGW tool.  Tony Reix (tony.reix(AT)laposte.net), Feb 20 2010
13347311 and 13372531 are found to be terms of this sequence (may be not the next ones) by Ryan Propper in September 2013.  Max Alekseyev, Oct 07 2013


REFERENCES

J. Brillhart et al., Factorizations of b^n + 1. Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 2nd edition, 1985; and later supplements.
N. J. A. Sloane, A Handbook of Integer Sequences, Academic Press, 1973 (includes this sequence).
N. J. A. Sloane and Simon Plouffe, The Encyclopedia of Integer Sequences, Academic Press, 1995 (includes this sequence).
S. S. Wagstaff, Jr., personal communication.


LINKS

Table of n, a(n) for n=1..41.
J. Brillhart et al., Factorizations of b^n + 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
C. Caldwell's The Top Twenty, Wagstaff.
C. Caldwell, New Mersenne Conjecture
H. Dubner, Generalized repunit primes, Math. Comp., 61 (1993), 927930. [Annotated scanned copy]
H. Dubner and T. Granlund, Primes of the Form (b^n+1)/(b+1), J. Integer Sequences, 3 (2000), #P00.2.7.
Editor's Note, Table of Wagstaff primes sent by D. H. Lehmer (annotated and scanned copy)
J. E. Foster, Problem 174, A solution in primes, Math. Mag., 27 (1954), 156157.
R. K. Guy, Letter to N. J. A. Sloane, Aug 1986
R. K. Guy, Letter to N. J. A. Sloane, 1987
H. Lifchitz, Mersenne and Fermat primes field
H. & R. Lifchitz, PRP Top Records.
Henri & Renaud Lifchitz, PRP Records.
Ryan Propper et al., New Wagstaff PRP exponents, mersenneforum.org
PRP top list: PRP top [From Tony Reix (tony.reix(AT)laposte.net), Feb 20 2010]
T. Reix, Some Maths about the VrbaReix PRP test [From Tony Reix (tony.reix(AT)laposte.net), Feb 20 2010]
S. S. Wagstaff, Jr., The Cunningham Project
Eric Weisstein's World of Mathematics, Repunit
Eric Weisstein's World of Mathematics, Wagstaff Prime
Eric Weisstein's World of Mathematics, Integer Sequence Primes
Wikipedia, Wagstaff prime
R. G. Wilson, v, Letter to N. J. A. Sloane, circa 1991.
Yahoo PrimeForm community: PrimeForm [From Tony Reix (tony.reix(AT)laposte.net), Feb 20 2010]


FORMULA

a(n) = A107036(n) for n>1.  Alexander Adamchuk, Feb 10 2007


MATHEMATICA

a = {}; Do[c = 1 + Sum[2^(2n  1), {n, 1, x}]; If[PrimeQ[c], AppendTo[a, c]], {x, 0, 100}]; a (* Artur Jasinski, Feb 09 2007 *)
Select[Range[5000], PrimeQ[(2^# + 1)/3] &] (* Michael De Vlieger, Jan 10 2018 *)


PROG

(PARI) forprime(p=2, 1e4, if(ispseudoprime(2^p\/3), print1(p", "))) \\ Charles R Greathouse IV, Jul 15 2011
(Haskell)
a000978 n = a000978_list !! (n1)
a000978_list = filter ((== 1) . a010051 . a001045) a065091_list
 Reinhard Zumkeller, Mar 24 2013
(Python)
from gmpy2 import divexact
from sympy import prime, isprime
A000978 = [p for p in (prime(n) for n in range(2, 10**2)) if isprime(divexact(2**p+1, 3))] # Chai Wah Wu, Sep 04 2014


CROSSREFS

Cf. A107036 = indices of prime Jacobsthal numbers.
Cf. A000979, A124400, A124401, A127955, A127956, A127957, A127958, A127936, A010051, A065091, A001045.
Sequence in context: A060770 A246568 A120334 * A128925 A204142 A131261
Adjacent sequences: A000975 A000976 A000977 * A000979 A000980 A000981


KEYWORD

hard,nonn,nice


AUTHOR

N. J. A. Sloane, Robert G. Wilson v


EXTENSIONS

a(30) from Kamil Duszenko (kdusz(AT)wp.pl), Feb 03 2003; a(30) was proved prime by Francois Morain with FastECPP.  Tony Reix (tony.reix(AT)laposte.net), Sep 03 2007
a(31)a(39) from Robert G. Wilson v, Apr 11 2005
a(40) from Vincent Diepeveen (diep(AT)xs4all.nl) added by Alexander Adamchuk, Jun 19 2008
a(41) from Tony Reix (tony.reix(AT)laposte.net), Feb 20 2010


STATUS

approved



