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A256762
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Numbers k such that R_k + 80 is prime, where R_k = 11...1 is the repunit (A002275) of length n.
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0
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3, 6, 27, 30, 108, 119, 209, 444, 1050, 15189, 16401, 81363, 134385, 135011
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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 (10^k + 719)/9 is prime.
Terms from Kamada data.
a(15) > 2.5*10^5.
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LINKS
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Table of n, a(n) for n=1..14.
Makoto Kamada, Near-repdigit numbers of the form AA...AABA.
Makoto Kamada, Prime numbers of the form 11...1191.
Index entries for primes involving repunits.
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EXAMPLE
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For k=3, R_3 + 80 = 111 + 80 = 191 which is prime.
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MATHEMATICA
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Select[Range[0, 250000], PrimeQ[(10^# + 719)/9] &]
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PROG
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(MAGMA) [n: n in [2..300] | IsPrime((10^n+719) div 9)]; // Vincenzo Librandi, Apr 10 2015
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CROSSREFS
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Cf. A002275.
Sequence in context: A128437 A200654 A208665 * A064283 A266857 A014561
Adjacent sequences: A256759 A256760 A256761 * A256763 A256764 A256765
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KEYWORD
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nonn,more,hard
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AUTHOR
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Robert Price, Apr 09 2015
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STATUS
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approved
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