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A256463
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Numbers k such that 3*R_k + 20 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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0
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1, 2, 3, 5, 24, 37, 53, 63, 84, 299, 506, 1687, 6660, 7473, 8754, 24048, 68078, 71223
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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 (10^k + 59)/3 is prime.
Terms from Kamada data. Note that Kamada does not recognize k=1 as 23 is a degenerate case of form AAA..ABA.
a(19) > 2.5*10^5.
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LINKS
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EXAMPLE
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For k=3, 3*R_3 + 20 = 333 + 20 = 353 which is prime.
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MATHEMATICA
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Select[Range[0, 250000], PrimeQ[(10^# + 59)/3] &]
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PROG
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(Magma) [n: n in [0..300] | IsPrime((10^n+59) div 3)]; // Vincenzo Librandi, Apr 11 2015
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CROSSREFS
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KEYWORD
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more,hard,nonn
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AUTHOR
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STATUS
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approved
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