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 A213790 Least number k such that 10^(n+k) - 10^n - 1 is prime. 4
 1, 10, 1, 2, 2, 3, 1, 6, 2, 20, 3, 4, 30, 9, 4, 5, 14, 4, 1, 658, 6, 10, 32, 9, 2, 9, 109, 8, 1, 7, 12, 6, 4, 2, 5, 137, 1, 15, 112, 30, 237, 83, 12, 21, 5, 4, 20, 15, 42, 3, 16, 41, 26, 60, 157, 8, 16, 76, 69, 10, 4, 4, 120, 39, 8, 7, 115, 22, 14, 2, 102 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,2 COMMENTS Near repdigit primes: concatenation of k-1 9's, one 8, and n 9's. LINKS Pierre CAMI, Table of n, a(n) for n = 1..1100 EXAMPLE (10^1-1)*10^1-1 = 89 prime so a(1) = 1. (10^10-1)*10^2-1 = 999999999899 prime so a(2) = 10. PROG (PARI) a(n) = {my(k=1); while (!ispseudoprime(10^(n+k) - 10^n - 1), k++); k; } \\ Michel Marcus, Sep 21 2019 CROSSREFS Sequence in context: A010180 A109013 A343102 * A240962 A324282 A085764 Adjacent sequences:  A213787 A213788 A213789 * A213791 A213792 A213793 KEYWORD nonn AUTHOR Pierre CAMI, Jun 20 2012 STATUS approved

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Last modified September 16 07:19 EDT 2021. Contains 347469 sequences. (Running on oeis4.)