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%I #20 Aug 06 2020 00:37:47
%S 5,13,6277,26777,27299,40031,44773,194119
%N Numbers n such that (43^n - 1)/42 is prime.
%C a(8) > 10^5. - _Robert Price_, Apr 12 2014
%H H. 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 H. Lifchitz, <a href="http://www.primenumbers.net/Henri/us/MersFermus.htm">Mersenne and Fermat primes field</a>
%p A240765:=n->`if`(isprime((43^n - 1)/42), n, NULL); seq(A240765(n), n=1..100000); # _Wesley Ivan Hurt_, Apr 12 2014
%t Select[Prime[Range[100000]], PrimeQ[(43^#-1)/42]&]
%o (PARI) is(n)=ispseudoprime((43^n-1)/42) \\ _Charles R Greathouse IV_, Feb 20 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.
%K hard,more,nonn
%O 1,1
%A _Robert Price_, Apr 12 2014
%E a(8) from _Paul Bourdelais_, Aug 04 2020