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A256933
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Numbers k such that R_(k+2) + 7*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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1, 2, 11, 62, 157, 170, 251, 500, 2275, 7525, 12230, 13658, 54727, 183227, 212779
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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 (163*10^k - 1)/9 is prime.
Terms from Kamada.
a(16) > 250000.
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LINKS
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EXAMPLE
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For k=2, R_4 + 7*10^2 = 1111 + 700 = 1811 which is prime.
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MATHEMATICA
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Select[Range[0, 250000], PrimeQ[(163*10^#-1)/9 ] &]
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PROG
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(Magma) [n: n in [0..300] | IsPrime((163*10^n-1) div 9)]; // Vincenzo Librandi, Apr 14 2015
(PARI) for(n=0, 300, if(isprime((163*10^n-1)/9), print1(n, ", "))) \\ Derek Orr, Apr 14 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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STATUS
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approved
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