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A056664
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Numbers k such that 80*R_k + 1 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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3
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2, 18, 78, 138, 222, 462, 543, 1095, 1418, 3246, 3876, 4416, 9506, 11090, 14601, 27810, 29187
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OFFSET
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1,1
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COMMENTS
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Also numbers k such that (8*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[ 80*(10^n - 1)/9 + 1 ], Print[n]], {n, 15000}]
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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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