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%I #30 May 05 2023 15:24:03
%S 3,11,17,1487,31013,48859,61403,472709,984349
%N Numbers k such that (20^k - 1)/19 is prime.
%H Paul Bourdelais, <a href="https://listserv.nodak.edu/cgi-bin/wa.exe?A2=NMBRTHRY;417ab0d6.0906">A Generalized Repunit Conjecture</a>
%H H. Dubner, <a href="http://dx.doi.org/10.1090/S0025-5718-1993-1185243-9">Generalized repunit primes</a>, Math. Comp., 61 (1993), 927-930.
%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)^n-1)/k
%t Select[Prime[Range[100]],PrimeQ[(20^#-1)/19]&]
%o (PARI) is(n)=isprime((20^n-1)/19) \\ _Charles R Greathouse IV_, Feb 17 2017
%K hard,more,nonn
%O 1,1
%A _Alexander Adamchuk_, Feb 11 2007
%E One more term from _Ryan Propper_, Mar 29 2007
%E a(5) & a(6) are probable primes found by _Paul Bourdelais_, Mar 05 2010
%E a(7) is a probable prime found by _Paul Bourdelais_, Mar 08 2010
%E a(8) is a probable prime found by _Paul Bourdelais_, Jan 15 2015
%E a(9) from _Paul Bourdelais_, Sep 13 2021
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