A057172 - OEIS

A057172

Numbers n such that (6^n + 1)/7 is a prime.

3, 11, 31, 43, 47, 59, 107, 811, 2819, 4817, 9601, 33581, 38447, 41341, 131891, 196337, 1313371

COMMENTS

a(15), a(16) and a(17) correspond to probable primes.

LINKS

J. Brillhart et al., Factorizations of b^n +- 1, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.

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

EXTENSIONS

a(12) was discovered by Kamil Duszenko, Jul 15 2003