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A056715
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Numbers k such that 5*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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0, 2, 8, 14, 17, 18, 33, 35, 126, 183, 324, 344, 866, 992, 1226, 2355, 13344, 75027, 77322
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OFFSET
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1,2
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COMMENTS
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Also numbers n such that (52*10^k - 7)/9 is prime.
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LINKS
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MATHEMATICA
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Do[ If[ PrimeQ[ 5*10^n + 7*(10^n-1)/9], 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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