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%I #19 Dec 23 2020 09:33:27
%S 3,19,373,419,491,1031,83497,223381
%N Numbers n such that (28^n+1)/29 is a prime.
%C (28^1031+1)/29, a 1491-digit number, has now been certified prime with Primo.
%C a(8) > 10^5. - _Robert Price_, Jul 10 2013
%H H. Dubner and T. Granlund, <a href="https://cs.uwaterloo.ca/journals/JIS/VOL3/DUBNER/dubner.html">Primes of the Form (b^n+1)/(b+1)</a>, J. Integer Sequences, 3 (2000), #P00.2.7.
%o (PARI) isok(n) = (denominator(p=(28^n+1)/29)==1) && isprime(p); \\ _Michel Marcus_, Oct 29 2017
%Y Cf. A071380, A071382.
%K nonn,more
%O 1,1
%A _Rick L. Shepherd_, May 22 2002
%E a(7) from _Robert Price_, Jul 10 2013
%E a(8)=223381 corresponds to a probable prime discovered by _Paul Bourdelais_, Dec 23 2020
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