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A056677
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Numbers k such that 20*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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3
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OFFSET
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1,2
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COMMENTS
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Also numbers k such that (2*10^(k+1)+43)/9 is prime.
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LINKS
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FORMULA
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MATHEMATICA
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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 *)
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CROSSREFS
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KEYWORD
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hard,nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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