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 A057186 Numbers n such that (20^n+1)/21 is a prime. 13
 5, 79, 89, 709, 797, 1163, 6971, 140053, 177967, 393257 (list; graph; refs; listen; history; text; internal format)
 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. 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

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