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A257027
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Numbers k such that 7*R_(k+2) - 6*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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0
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0, 2, 3, 9, 11, 18, 74, 131, 144, 161, 224, 282, 390, 398, 614, 791, 1313, 1866, 9708, 10544, 13292, 13394, 29703, 30779, 72446
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OFFSET
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1,2
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COMMENTS
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Also, numbers k such that (646*10^k - 7)/9 is prime.
Terms from Kamada.
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LINKS
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EXAMPLE
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For k=2, 7*R_4 - 6*10^2 = 7777 - 600 = 7177 which is prime.
a(1) =0 associated with 71. a(2)=2 associated with 7177. a(3)=3 associated with 71777. a(4) = 9 associated with 71777777777 etc . - Robert Price, Jul 31 2016
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(646*10^#-7)/9 ] &]
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PROG
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(PARI) for(n=0, 200, if(isprime((646*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..300] | IsPrime((646*10^n-7) div 9)]; // Vincenzo Librandi, Apr 15 2015
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CROSSREFS
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KEYWORD
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more,hard,nonn
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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