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A216888
Numbers n such that 6*3^n + 1 is prime.
0
0, 1, 3, 4, 5, 8, 15, 16, 29, 53, 56, 59, 64, 131, 179, 319, 695, 781, 821, 896, 1251, 1453, 4216, 5479, 6224, 7841, 12095, 13781, 17719, 43955, 64821, 82779, 105105, 152528, 165895, 191813, 529679, 1074725, 1086111, 1175231
OFFSET
1,3
FORMULA
a(n) = A003306(n+1)-1. - Bruno Berselli, Sep 27 2012
EXAMPLE
3 is in the sequence because 6*3^3 + 1 = 163 is prime;
7 is not in the sequence because 6*3^7 + 1 = 13123 = 11*1193 is composite.
MATHEMATICA
Select[Range[5000], PrimeQ[6 3^# + 1] &]
PROG
(Magma) /* Gives only the terms up to 1453: */ [n: n in [0..1500] | IsPrime(6*3^n + 1)];
(PARI) is(n)=ispseudoprime(6*3^n+1) \\ Charles R Greathouse IV, Jun 13 2017
CROSSREFS
Associated primes are in A111974.
Sequence in context: A039020 A055742 A263041 * A362218 A106048 A219038
KEYWORD
nonn,hard
AUTHOR
Vincenzo Librandi, Sep 26 2012
EXTENSIONS
More terms from Vincenzo Librandi, Oct 01 2012
STATUS
approved