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

3, 11, 17, 43, 271, 156217, 328129

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