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A256724
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Numbers k such that R_k + 30 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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0
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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 + 269)/9 is prime.
Terms from Kamada data. Note that Kamada does not recognize k=1 as 31 is a degenerate case of form AAA..ABA.
a(7) > 10^6.
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LINKS
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EXAMPLE
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For k=8, R_8 + 30 = 11111111 + 30 = 11111141 which is prime.
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MATHEMATICA
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Select[Range[0, 250000], PrimeQ[(10^# + 269)/9] &]
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PROG
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(Magma) [n: n in [1..400] | IsPrime((10^n+269) div 9)]; // Vincenzo Librandi, Apr 10 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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