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A056684
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Numbers k such that 50*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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OFFSET
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1,1
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COMMENTS
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Also numbers k such that (5*10^(k+1)-41)/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[50*(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,bref,more
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AUTHOR
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STATUS
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approved
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