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A056682
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Numbers k such that 40*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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3
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0, 1, 3, 9, 19, 25, 721, 1309, 3169, 28933, 66283, 67795
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OFFSET
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1,3
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COMMENTS
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Also numbers k such that (4*10^(k+1)+23)/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[40*(10^n - 1)/9 + 7], Print[n]], {n, 0, 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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