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A256828
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Numbers k such that 7*R_k - 40 is prime, where R_k = 11...1 is the repunit (A002275) of length k.
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0
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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 (7*10^k - 367)/9 is prime.
Terms from Kamada.
a(9) > 30000.
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LINKS
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Table of n, a(n) for n=1..8.
Makoto Kamada, Near-repdigit numbers of the form AA...AABA.
Makoto Kamada, Prime numbers of the form 77...7737.
Index entries for primes involving repunits.
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EXAMPLE
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For k=6, 7*R_6 - 40 = 777777 - 40 = 777737 which is prime.
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MATHEMATICA
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Select[Range[0, 250000], PrimeQ[(7*10^# - 367)/9] &]
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PROG
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(Magma) [n: n in [2..300] | IsPrime((7*10^n-367) div 9)]; // Vincenzo Librandi, Apr 11 2015
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CROSSREFS
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Cf. A002275.
Sequence in context: A218759 A295499 A059413 * A197055 A258625 A062026
Adjacent sequences: A256825 A256826 A256827 * A256829 A256830 A256831
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KEYWORD
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more,hard,nonn
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AUTHOR
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Robert Price, Apr 10 2015
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STATUS
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approved
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