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A056657
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Numbers k such that 60*R_k + 7 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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6
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0, 1, 5, 7, 8, 10, 19, 22, 40, 62, 65, 118, 121, 148, 251, 283, 304, 591, 745, 874, 1203, 1363, 2239, 2402, 5105, 5775, 5812, 12455, 14234, 39605, 55543, 84238, 275921
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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 (20*10^k+1)/3 is prime.
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LINKS
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FORMULA
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EXAMPLE
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7, 67, 666667, 66666667, 666666667, 66666666667, etc. are primes.
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MATHEMATICA
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Do[ If[ PrimeQ[ 60*(10^n - 1)/9 + 7 ], Print[n]], {n, 25556}]
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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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39605 and 55543 from Serge Batalov, Jun 2009
84238 from Serge Batalov, Jul 06 2009 confirmed as next term by Ray Chandler, Feb 23 2012
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STATUS
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approved
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