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 A000978 Wagstaff numbers: numbers k such that (2^k + 1)/3 is prime. (Formerly M2413 N0956) 83
 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 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS It is easy to see that the definition implies that k 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^(2i-1). - 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(p-1) == S(0) (mod W(p)). - Tony Reix, 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 Vrba-Reix 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, Feb 20 2010 13347311 and 13372531 were found to be terms of this sequence (maybe not the next ones) by Ryan Propper in September 2013. - Max Alekseyev, Oct 07 2013 The next term is larger than 10 million. - Gord Palameta, Mar 22 2019 Ryan Propper found another likely term, 15135397, though it only corresponds to a probable prime. - Charles R Greathouse IV, Jul 01 2021 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), 927-930. [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), 156-157. 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. Gord Palameta, There are no new Wagstaff primes with exponent below 10 million, mersenneforum.org Ryan Propper et al., New Wagstaff PRP exponents, mersenneforum.org PRP top list: PRP top [From Tony Reix, Feb 20 2010] Tony Reix, Yahoo Primeform Group Message 10184 dd. Feb 20, 2010, reconstruction in html. T. Reix, Some Maths about the Vrba-Reix PRP test [From Tony Reix, 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. FORMULA a(n) = A107036(n) for n>1. - Alexander Adamchuk, Feb 10 2007 MATHEMATICA Select[Range[5000], PrimeQ[(2^# + 1)/3] &] (* Michael De Vlieger, Jan 10 2018 *) Select[Prime[Range[2, 500]], PrimeQ[(2^#+1)/3]&] (* Harvey P. Dale, Jun 13 2022 *) PROG (PARI) forprime(p=2, 5000, if(ispseudoprime(2^p\/3), print1(p", "))) \\ Charles R Greathouse IV, Jul 15 2011 (Haskell) a000978 n = a000978_list !! (n-1) 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: A246568 A338132 A120334 * A128925 A204142 A131261 Adjacent sequences: A000975 A000976 A000977 * A000979 A000980 A000981 KEYWORD nonn,hard,nice,more 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, 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, Feb 20 2010 STATUS approved

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