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A099415
Numbers k such that 5*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
OFFSET
1,1
COMMENTS
Also numbers k such that (5*10^k - 41)/9 is prime.
a(4) > 10^5. - Robert Price, Nov 16 2014
FORMULA
a(n) = A056684(n) + 1. - Robert Price, Nov 17 2014
MAPLE
A099415:=n->`if`(isprime((5*10^n-41)/9), n, NULL): seq(A099415(n), n=1..10^3); # Wesley Ivan Hurt, Nov 16 2014
MATHEMATICA
Do[ If[ PrimeQ[ 5(10^n - 1)/9 - 4], Print[n]], {n, 15000}]
Select[Range[10000], PrimeQ[(5 10^# - 41) / 9] &] (* Vincenzo Librandi, Nov 17 2014 *)
PROG
(Magma) [n: n in [0..500] | IsPrime((5*10^n-41) div 9)]; // Vincenzo Librandi, Nov 17 2014
CROSSREFS
Sequence in context: A041310 A275959 A066321 * A042293 A061097 A121288
KEYWORD
nonn,bref,more
AUTHOR
Robert G. Wilson v, Oct 14 2004
STATUS
approved