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A056659
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Numbers k such that 10*R_k + 9 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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1, 4, 5, 7, 16, 49, 683, 719, 1451, 1678, 3145, 72820
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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+1)+71)/9 is prime.
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LINKS
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FORMULA
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MATHEMATICA
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Do[ If[ PrimeQ[10*(10^n - 1)/9 + 9], Print[n]], {n, 5000}]
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CROSSREFS
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KEYWORD
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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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