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A056681
Numbers k such that 40*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
2
1, 3, 10, 27, 54, 93, 474, 2079, 4834, 5844, 12337, 20193, 49528, 56988, 71545, 77308
OFFSET
1,2
COMMENTS
Also numbers k such that (4*10^(k+1)-31)/9 is prime.
FORMULA
a(n) = A099412(n+1) - 1. - Robert Price, Jan 30 2015
MATHEMATICA
Do[ If[ PrimeQ[40*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]
CROSSREFS
Sequence in context: A196232 A282282 A105660 * A192959 A100624 A320586
KEYWORD
hard,nonn
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(11)-a(16) derived from A099412 by Robert Price, Jan 30 2015
STATUS
approved