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A056696
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Numbers k such that 90*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, 4, 6, 32, 44, 104, 196, 198, 280, 300, 316, 1106, 1656, 3394, 35924, 37596, 64304, 80138, 221630
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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 10^(k+1) - 9 is a prime.
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LINKS
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FORMULA
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MATHEMATICA
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Do[ If[ PrimeQ[90*(10^n - 1)/9 + 1], Print[n]], {n, 0, 7000}]
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CROSSREFS
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KEYWORD
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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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