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A257028
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Numbers k such that 7*R_(k+2) - 5*10^k is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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2
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1, 4, 10, 28, 49, 64, 79, 109, 169, 1270, 5638, 6812, 7951, 11737, 16360, 22840, 25394, 33394
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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 (655*10^k - 7)/9 is prime.
Terms from Kamada.
a(18) > 30000.
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LINKS
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EXAMPLE
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For k=4, 7*R_6 - 5*10^2 = 777777 - 50000 = 727777 which is prime.
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MATHEMATICA
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Select[Range[0, 30000], PrimeQ[(655*10^#-7)/9 ] &]
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PROG
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(PARI) for(n=0, 200, if(isprime((655*10^n-7)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 2015
(Magma) [n: n in [0..400] | IsPrime((655*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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