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A103054
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Numbers n such that 7*10^n + 2*R_n + 7 is prime, where R_n = 11...1 is the repunit (A002275) of length n.
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1
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1, 3, 4, 7, 9, 13, 33, 37, 76, 147, 297, 1851, 3457, 3609, 45219
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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 (65*10^n+61)/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[(65*10^n + 61)/9], Print[n]], {n, 0, 10000}]
Select[Range[10000], PrimeQ[(65 10^# + 61)/9] &] (* Vincenzo Librandi, Oct 02 2015 *)
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PROG
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(Magma) [n: n in [0..3*10^2]| IsPrime((65*10^n+61) div 9)]; // Vincenzo Librandi, Oct 02 2015
(PARI) for(n=1, 1e4, if (isprime((65*10^n+61)/9), print1(n", "))) \\ Altug Alkan, Oct 02 2015
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CROSSREFS
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KEYWORD
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more,nonn
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AUTHOR
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EXTENSIONS
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a(15) from Erik Branger May 01 2013 by Ray Chandler, Aug 16 2013
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STATUS
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approved
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