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

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

Offset

1,1

Comments

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

Links

Table of n, a(n) for n=1..10.

H. Dubner and T. Granlund, Primes of the Form (b^n+1)/(b+1), J. Integer Sequences, 3 (2000), #P00.2.7.

Paul Bourdelais, A Generalized Repunit Conjecture.

Prog

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

Crossrefs

Sequence in context: A015973 A363376 A136578 * A106939 A142114 A023148

Adjacent sequences: A057183 A057184 A057185 * A057187 A057188 A057189

Keyword

nonn

Author

N. J. A. Sloane, Sep 15 2000

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

Status

approved