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 A230036 Numbers n such that (39^n + 1)/40 is prime. 6
 3, 13, 149, 15377 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS All terms are primes. a(5) > 10^5. LINKS 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 Do[ p=Prime[n]; If[ PrimeQ[ (39^p + 1)/40 ], Print[p] ], {n, 1, 9592} ] PROG (PARI) is(n)=ispseudoprime((39^n+1)/40) \\ Charles R Greathouse IV, Feb 17 2017 CROSSREFS Cf. A000978 (numbers n such that (2^n + 1)/3 is prime). Cf. A007658, A057171, A057172, A057173, A057175, A001562, A057177, A057178, A057179, A057180, A057181, A057182, A057183, A057184, A057185, A057186, A057187, A057188, A057189, A057190, A057191, A071380, A071381, A071382, A084741, A084742, A065507, A126659, A126856, A185240, A229145, A229524. Sequence in context: A287029 A339025 A317074 * A014376 A224990 A065622 Adjacent sequences:  A230033 A230034 A230035 * A230037 A230038 A230039 KEYWORD hard,more,nonn AUTHOR Robert Price, Oct 05 2013 STATUS approved

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Last modified August 9 00:15 EDT 2022. Contains 356016 sequences. (Running on oeis4.)