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A092480
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Primes of the form 40*R_k + 7, where R_k is the repunit (A002275) of length k.
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2
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OFFSET
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1,1
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COMMENTS
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LINKS
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MATHEMATICA
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Select[Table[(((4*10^n-31)/ 9)+6), {n, 1, 800}], PrimeQ] (* Vincenzo Librandi, Dec 13 2011 *)
Select[Table[FromDigits[PadLeft[{7}, n, 4]], {n, 30}], PrimeQ] (* Harvey P. Dale, Jan 19 2020 *)
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PROG
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(Magma) [a: n in [1..720] | IsPrime(a) where a is ((4*10^n-31) div 9)+6 ]; // Vincenzo Librandi, Dec 13 2011
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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