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

3, 11, 17, 43, 271, 156217, 328129, 3078871

Offset

1,1

Links

Table of n, a(n) for n=1..8.

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) from Paul Bourdelais, Apr 26 2010

a(7) from Paul Bourdelais, Jun 02 2014

a(8) from Paul Bourdelais, Jun 01 2026

Status

approved