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A259120
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Numbers k such that 9*R_k + 10^k - 8 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
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0
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1, 2, 4, 8, 40, 86, 200, 730, 1460, 23672, 28630
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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 2*10^k - 9 is prime.
Terms from Kamada data.
a(12) > 2*10^5.
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LINKS
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EXAMPLE
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For k=4, 9*R_4 + 10^k - 2 = 9999 + 10000 - 8 = 19991 which is prime.
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MATHEMATICA
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Select[Range[200000], PrimeQ[2*10^n-9] &]
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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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