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%I #35 Sep 10 2018 13:26:47
%S 5,79,89,709,797,1163,6971,140053,177967,393257
%N Numbers n such that (20^n+1)/21 is a prime.
%C (20^1163+1)/21, a 1512-digit number, has been certified prime with Primo. - _Rick L. Shepherd_, May 17 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 Paul Bourdelais, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a>.
%o (PARI) forprime(n=3,10^7, if( ispseudoprime((20^n+1)/21), print1(n,", "))); \\ _Joerg Arndt_, May 26 2013
%K nonn
%O 1,1
%A _N. J. A. Sloane_, Sep 15 2000
%E a(8) from _Paul Bourdelais_, Apr 06 2010
%E a(9) from _Paul Bourdelais_, May 26 2013
%E a(10), corresponding to a probable prime, from _Paul Bourdelais_, Dec 08 2014
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