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A259126
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Numbers k such that 5*R_k + 3*10^k - 2 is prime, where R_k = 11...11 is the repunit (A002275) of length k.
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0
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2, 8, 140, 230, 426, 462, 726, 1974, 7230, 45860, 47304
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OFFSET
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1,1
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COMMENTS
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Also, numbers k such that (32*10^k - 23)/9 is prime.
Terms from Kamada data.
a(12) > 10^5.
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LINKS
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EXAMPLE
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For k=2, 5*R_2 + 3*10^k - 2 = 55 + 300 - 2 = 353 which is prime.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(32*10^#-23)/9] &]
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PROG
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(Magma) [n: n in [0..450] | IsPrime((32*10^n-23) div 9)]; // Vincenzo Librandi, Jun 19 2015
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CROSSREFS
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KEYWORD
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nonn,more,hard
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AUTHOR
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STATUS
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approved
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