%I
%S 3,11,17,43,271,156217,328129
%N Numbers n such that (21^n  1)/20 is prime.
%H Paul Bourdelais,<a href="https://listserv.nodak.edu/cgibin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a>
%H H. Dubner, <a href="http://dx.doi.org/10.1090/S00255718199311852439">Generalized repunit primes</a>, Math. Comp., 61 (1993), 927930.
%H H. Lifchitz, <a href="http://www.primenumbers.net/Henri/us/MersFermus.htm">Mersenne and Fermat primes field</a>
%H <a href="/index/Pri#primepop">Index to primes in various ranges</a>, form ((k+1)^n1)/k
%t Select[Prime[Range[100]],PrimeQ[(21^#1)/20]&]
%o (Prime95) PRP=1,21,328129,1,0,0,"20"
%o (PARI) is(n)=isprime((21^n1)/20) \\ _Charles R Greathouse IV_, Feb 17 2017
%K hard,more,nonn
%O 1,1
%A _Alexander Adamchuk_, Feb 11 2007
%E a(6)=156217 is a probable prime discovered by _Paul Bourdelais_, Apr 26 2010
%E a(7)=328129 is a probable prime discovered by _Paul Bourdelais_, Jun 02 2014
