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A056685
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Numbers k such that 50*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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0, 1, 7, 25, 65, 73, 232, 472, 539, 2773, 64714
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OFFSET
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1,3
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COMMENTS
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Also numbers k such that (5*10^(k+1)-23)/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[50*(10^n - 1)/9 + 3], Print[n]], {n, 0, 5000}]
Select[Range[0, 3000], PrimeQ[(5 10^(# + 1) - 23) / 9] &] (* Vincenzo Librandi, Nov 14 2014 *)
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PROG
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(Magma) [n: n in [0..400]| IsPrime((5*10^(n+1)-23)div 9)]; // Vincenzo Librandi, Nov 14 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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STATUS
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approved
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