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A056661
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Numbers k such that 40*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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0, 1, 2, 5, 8, 11, 29, 31, 182, 296, 491, 41315
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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)-13)/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 + 3], Print[n]], {n, 0, 5000}]
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CROSSREFS
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KEYWORD
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hard,more,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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