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A127996 Numbers n such that (21^n - 1)/20 is prime. 10


%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/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[(21^#-1)/20]&]

%o (Prime95) PRP=1,21,328129,-1,0,0,"20"

%o (PARI) is(n)=isprime((21^n-1)/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

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Last modified January 18 19:40 EST 2020. Contains 331029 sequences. (Running on oeis4.)