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A099420
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Numbers k such that 7*R_k - 4 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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3
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1, 2, 3, 5, 9, 12, 15, 21, 264, 383, 2720, 4494, 21167, 45825, 55851, 64568, 70727
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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 (7*10^k - 43)/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[ 7(10^n - 1)/9 - 4], Print[n]], {n, 0, 5000}]
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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