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 A127996 Numbers n such that (21^n - 1)/20 is prime. 10
 3, 11, 17, 43, 271, 156217, 328129 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 LINKS Paul Bourdelais,A Generalized Repunit Conjecture H. Dubner, Generalized repunit primes, Math. Comp., 61 (1993), 927-930. H. Lifchitz, Mersenne and Fermat primes field Index to primes in various ranges, form ((k+1)^n-1)/k MATHEMATICA Select[Prime[Range[100]], PrimeQ[(21^#-1)/20]&] PROG (Prime95) PRP=1, 21, 328129, -1, 0, 0, "20" (PARI) is(n)=isprime((21^n-1)/20) \\ Charles R Greathouse IV, Feb 17 2017 CROSSREFS Sequence in context: A176804 A078116 A245045 * A032008 A061368 A145701 Adjacent sequences:  A127993 A127994 A127995 * A127997 A127998 A127999 KEYWORD hard,more,nonn AUTHOR Alexander Adamchuk, Feb 11 2007 EXTENSIONS a(6)=156217 is a probable prime discovered by Paul Bourdelais, Apr 26 2010 a(7)=328129 is a probable prime discovered by Paul Bourdelais, Jun 02 2014 STATUS approved

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Last modified December 13 17:17 EST 2019. Contains 329970 sequences. (Running on oeis4.)