

A057186


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


13




OFFSET

1,1


COMMENTS

(20^1163+1)/21, a 1512digit 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: A214443 A015973 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



