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A056727
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Numbers k such that 9*10^k + 7*R_k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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1
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1, 2, 4, 19, 28, 73, 203, 220, 274, 292, 470, 763, 1891, 3307, 7007, 7306, 9755, 11395, 39452, 78242
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OFFSET
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1,2
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COMMENTS
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Also numbers k such that (88*10^k - 7)/9 is prime.
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LINKS
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MATHEMATICA
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Do[ If[ PrimeQ[ 9*10^n + 7*(10^n-1)/9], Print[n]], {n, 0, 10000}]
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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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