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A056689
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Numbers k such that 70*R_k + 3, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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0, 1, 2, 4, 8, 11, 14, 20, 263, 382, 2719, 4493, 21166, 45824, 55850, 64567, 70726
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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 (7*10^(k+1)-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[70*(10^n - 1)/9 + 3], 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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