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%I #28 Aug 20 2021 10:02:39
%S 3,5,7,13,37,347,17597,59183,80761,210599,394579
%N Numbers n such that (21^n+1)/22 is a prime.
%C Some of the larger terms correspond to probable primes.
%H H. Dubner and T. Granlund, <a href="http://www.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>
%H Paul Bourdelais, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a> [From _Paul Bourdelais_, Mar 24 2010]
%o (Prime95) PRP=1,21,394579,1,0,0,"22"
%K nonn
%O 1,1
%A _N. J. A. Sloane_, Sep 15 2000
%E a(7) from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 05 2008
%E a(8) corresponds to a probable prime discovered by _Paul Bourdelais_, Mar 22 2010
%E a(9) corresponds to a probable prime discovered by _Paul Bourdelais_, Mar 24 2010
%E a(10) corresponds to a probable prime discovered by _Paul Bourdelais_, Aug 30 2013
%E a(11) corresponds to a probable prime discovered by _Paul Bourdelais_, Oct 23 2018