A057186 Numbers k such that (20^k+1)/21 is a prime.

5, 79, 89, 709, 797, 1163, 6971, 140053, 177967, 393257, 463511

Offset: 1

Author

N. J. A. Sloane, Sep 15 2000

Keywords

• nonn

Comments

(20^1163+1)/21, a 1512-digit number, has been certified prime with Primo. - Rick L. Shepherd, May 17 2002

Links

• Paul Bourdelais, A Generalized Repunit Conjecture.
• H. Dubner and T. Granlund, Primes of the Form (b^n+1)/(b+1), J. Integer Sequences, 3 (2000), #P00.2.7.

Programs

PARI

forprime(n=3,10^7, if( ispseudoprime((20^n+1)/21), print1(n,", "))); \\ Joerg Arndt, May 26 2013

Extensions

a(8) from Paul Bourdelais, Apr 06 2010

a(9) from Paul Bourdelais, May 26 2013

a(10), corresponding to a probable prime, from Paul Bourdelais, Dec 08 2014

a(11) from Paul Bourdelais, Mar 10 2025