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A056677
Numbers k such that 20*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
3
0, 2, 8, 14, 27, 63, 1167, 1694, 2361, 116619
OFFSET
1,2
COMMENTS
Also numbers k such that (2*10^(k+1)+43)/9 is prime.
FORMULA
a(n) = A099409(n+1) - 1. - Robert Price, Jan 30 2015
MATHEMATICA
Do[ If[ PrimeQ[20*(10^n - 1)/9 + 7], Print[n]], {n, 0, 5000}]
Flatten[Position[Table[FromDigits[PadLeft[{7}, n, 2]], {n, 5000}], _?PrimeQ]]-1 (* Harvey P. Dale, May 01 2012 *)
Select[Range[0, 2500], PrimeQ[20*(10^n - 1)/9 + 7]&] (* Mikk Heidemaa, Nov 28 2015 *)
CROSSREFS
KEYWORD
hard,nonn,more
AUTHOR
Robert G. Wilson v, Aug 10 2000
EXTENSIONS
a(10) from Kamada link entered by Michael S. Branicky, Jan 14 2023
STATUS
approved