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A056660
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Numbers k such that 20*R_k + 1 is a prime, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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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 (2*10^(k+1)-11)/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[ 20*(10^n - 1)/9 + 1 ], Print[n]], {n, 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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More terms from Herman Jamke (hermanjamke(AT)fastmail.fm), Jan 01 2008
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STATUS
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approved
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