A245442 - OEIS

%I #14 Aug 04 2020 12:17:07

%S 3,5,127,139,347,661,2203,6521,210319

%N Numbers n such that (50^n - 1)/49 is prime.

%C a(9) > 10^5.

%C All terms are prime.

%H Harvey Dubner, <a href="http://dx.doi.org/10.1090/S0025-5718-1993-1185243-9">Generalized repunit primes</a>, Math. Comp., 61 (1993), 927-930.

%H Henri Lifchitz, <a href="http://www.primenumbers.net/Henri/us/MersFermus.htm">Mersenne and Fermat primes field</a>

%p A245442:=n->`if`(isprime((50^n - 1)/49), n, NULL); seq(A245442(n), n=1..100000); # _Wesley Ivan Hurt_, Apr 12 2014

%t Select[Prime[Range[100000]], PrimeQ[(50^# - 1)/49] &]

%o (PARI) is(n)=ispseudoprime((50^n-1)/49) \\ _Charles R Greathouse IV_, Jun 13 2017

%Y Cf. A028491, A004061, A004062, A004063, A004023, A005808, A004064, A016054, A006032, A006033, A006034, A006035, A127995, A127996, A127997, A127998, A127999, A098438, A128002, A128003, A128004, A128005, A240765, A242797, A243279, A245237.

%K hard,more,nonn

%O 1,1

%A _Robert Price_, Jul 22 2014

%E a(9)=210319 corresponds to a probable prime discovered by _Paul Bourdelais_, Aug 04 2020