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A259118
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Numbers k such that 6*R_k + 10^k - 5 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
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0
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1, 4, 12, 16, 18, 36, 52, 72, 100, 6232, 24028, 40222, 66394
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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 (5*10^k - 17)/3 is prime.
Terms from Kamada data.
a(14) > 2*10^5.
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LINKS
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EXAMPLE
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For k=4, 6*R_4 + 10^k - 5 = 6666 + 10000 - 5 = 16661 which is prime.
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MATHEMATICA
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Select[Range[0, 200000], PrimeQ[(5*10^#-17)/3] &]
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PROG
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(Magma) [n: n in [0..200] | IsPrime((5*10^n-17) div 3)]; // Vincenzo Librandi, Jun 19 2015
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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STATUS
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approved
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