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 A057172 Numbers n such that (6^n + 1)/7 is a prime. 13
 3, 11, 31, 43, 47, 59, 107, 811, 2819, 4817, 9601, 33581, 38447, 41341, 131891, 196337, 1313371 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS a(15), a(16) and a(17) correspond to probable primes. LINKS P. Bourdelais,A Generalized Repunit Conjecture J. Brillhart et al., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002. H. Dubner and T. Granlund, Primes of the Form (b^n+1)/(b+1), J. Integer Sequences, 3 (2000), #P00.2.7. H. Lifchitz, Mersenne and Fermat primes field Eric Weisstein's World of Mathematics, Repunit MATHEMATICA Select[Range[5000], PrimeQ[(6^# + 1) / 7] &] (* Vincenzo Librandi, Oct 29 2017 *) PROG (PARI) isok(n) = (denominator(p=(6^n+1)/7)==1) && isprime(p); \\ Michel Marcus, Oct 29 2017 CROSSREFS Sequence in context: A018781 A119215 A266970 * A277167 A277049 A261148 Adjacent sequences:  A057169 A057170 A057171 * A057173 A057174 A057175 KEYWORD nonn,more AUTHOR N. J. A. Sloane, Sep 15 2000 EXTENSIONS a(12) was discovered by Kamil Duszenko, Jul 15 2003 a(13) was discovered by Henri Lifchitz, Sep 15 2007 a(14) was discovered by Paul Bourdelais, Oct 01 2007 a(15) was discovered by Paul Bourdelais, Feb 01 2010 a(16) was discovered by Paul Bourdelais, Feb 19 2010 a(17) was discovered by Paul Bourdelais, Jan 28 2019 STATUS approved

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Last modified May 21 03:23 EDT 2019. Contains 323434 sequences. (Running on oeis4.)