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%I #33 Sep 10 2018 13:20:52
%S 3,7,29,1091,2423,54449,67489,551927
%N Numbers n such that (15^n + 1)/16 is a prime.
%C a(6), a(7) and a(8) correspond to probable primes.
%H Paul Bourdelais, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a>
%H J. Brillhart et al., <a href="http://dx.doi.org/10.1090/conm/022">Factorizations of b^n +- 1</a>, Contemporary Mathematics, Vol. 22, Amer. Math. Soc., Providence, RI, 3rd edition, 2002.
%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.
%H H. Lifchitz, <a href="http://www.primenumbers.net/Henri/us/MersFermus.htm">Mersenne and Fermat primes field</a>
%t Select[Range[3000], PrimeQ[(15^# + 1) / 16] &] (* _Vincenzo Librandi_, Oct 29 2017 *)
%o (Prime95) PRP=1,15,551927,1,0,0,"16"
%o (PARI) isok(n) = (denominator(p=(15^n+1)/16)==1) && isprime(p); \\ _Michel Marcus_, Oct 29 2017
%K nonn,more
%O 1,1
%A _N. J. A. Sloane_, Sep 15 2000
%E a(6) from _Paul Bourdelais_, Mar 15 2010
%E a(7) from _Paul Bourdelais_, Mar 16 2010
%E a(8) from _Paul Bourdelais_, Jul 03 2013
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