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A057186
Numbers n such that (20^n+1)/21 is a prime.
14
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
H. Dubner and T. Granlund, Primes of the Form (b^n+1)/(b+1), J. Integer Sequences, 3 (2000), #P00.2.7.
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
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