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A056656
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Numbers k such that 20*R_k + 3 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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0, 1, 2, 7, 10, 35, 94, 100, 127, 259, 350, 466, 644, 1010, 1177, 1216, 2441, 3760, 3805, 15616, 26458, 63116, 88544, 93496
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OFFSET
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1,3
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COMMENTS
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Also numbers k such that (2*10^(k+1)+7)/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[ 20*(10^n - 1)/9 + 3 ], 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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15616 and 26458 from Erik Branger, Jan 31 2010
63116, 88544 and 93496 from Erik Branger, Mar 14 2011; confirmed as next terms by Ray Chandler, Feb 17 2012
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STATUS
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approved
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