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A056693
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Numbers k such that 70*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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1, 65, 85, 89, 101, 385, 623, 7783, 18535, 113756, 135878
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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 (7*10^(k+1)+11)/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[70*(10^n - 1)/9 + 9], Print[n]], {n, 0, 5000}]
Select[Range[700], PrimeQ[(7 10^(# + 1) + 11) / 9] &] (* Vincenzo Librandi, Nov 22 2014 *)
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PROG
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(Magma) [n: n in [1..400] | IsPrime((7*10^(n+1)+11) div 9)]; Vincenzo Librandi, Nov 22 2014
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CROSSREFS
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KEYWORD
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hard,nonn
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AUTHOR
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EXTENSIONS
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More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
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STATUS
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approved
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