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A257041
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Numbers k such that 9*R_(k+2) - 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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3, 4, 5, 9, 19, 25, 34, 70, 82, 718, 2035, 2494, 3369, 3953, 4219, 8524, 30943, 49375, 52082, 139670, 303255
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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 99*10^k - 1 is prime.
Terms from Kamada.
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LINKS
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EXAMPLE
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For k=3, 9*R_5 - 10^3 = 99999 - 1000 = 98999 which is prime.
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MATHEMATICA
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Select[Range[0, 280000], PrimeQ[99*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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EXTENSIONS
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STATUS
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approved
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