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A257039
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Numbers k such that 9*R_(k+2) - 4*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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0
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10, 12, 63, 69, 156, 328, 340, 344, 444, 672, 894, 1464, 1670, 1708, 2010, 4306, 7888, 8864, 9478, 9621, 26004, 36992, 71600, 98738, 118949, 130565, 140326, 183452, 211983, 225618
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OFFSET
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1,1
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COMMENTS
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Also, numbers k such that 96*10^k - 1 is prime.
Terms from Kamada.
846519 reported to Kamada by Bruno DallOsto is also in this sequence. It may or may not be a(31).
a(31) > 230000.
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LINKS
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EXAMPLE
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For k=10, 9*R_12 - 4*10^10 = 999999999999 - 40000000000 = 959999999999 which is prime.
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MATHEMATICA
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Select[Range[0, 230000], PrimeQ[96*10^#-1 ] &]
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PROG
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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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