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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
OFFSET
1,2
COMMENTS
Near repdigit primes: concatenation of k-1 9's, one 8, and n 9's.
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.
MATHEMATICA
lnk[n_]:=Module[{k=1, c=10^n+1}, While[!PrimeQ[10^(n+k)-c], k++]; k]; Array[lnk, 80] (* Harvey P. Dale, Aug 12 2024 *)
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 A376301 A324282
KEYWORD
nonn
AUTHOR
Pierre CAMI, Jun 20 2012
STATUS
approved