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A056683
Numbers k such that 40*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
2, 4, 5, 47, 107, 244, 1043, 20207, 52739, 89188
OFFSET
1,1
COMMENTS
Also numbers k such that (4*10^(k+1) + 41)/9 is prime.
a(11) > 10^5. - Robert Price, Nov 09 2014
FORMULA
a(n) = A099414(n+1) - 1. - Robert Price, Nov 09 2014
MATHEMATICA
Do[ If[ PrimeQ[40*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]
PROG
(Magma) [n: n in [0..300] | IsPrime((4*10^(n+1)+41)div 9)]; Vincenzo Librandi, Nov 10 2014
CROSSREFS
Sequence in context: A154775 A210418 A167454 * A065574 A247963 A105288
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(8)-a(10) derived from A099414 by Robert Price, Nov 09 2014
STATUS
approved