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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

Table of n, a(n) for n=1..7.

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.)