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A288823
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Numbers k such that (86*10^k - 221)/9 is prime.
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0
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1, 4, 7, 13, 17, 31, 47, 64, 226, 251, 268, 310, 352, 394, 478, 599, 1529, 1679, 11590, 12922, 13151, 18808, 47188, 52450, 83038, 93217, 128086, 154853, 175774
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OFFSET
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1,2
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COMMENTS
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For k>1, numbers such that the digit 9 followed by k-2 occurrences of the digit 5 followed by the digits 31 is prime (see Example section).
a(39) > 2*10^5.
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LINKS
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Table of n, a(n) for n=1..29.
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 95w31.
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EXAMPLE
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4 is in this sequence because (86*10^4 - 221)/9 = 95531 is prime.
Initial terms and primes associated:
a(1) = 1, 71;
a(2) = 4, 95531;
a(3) = 7, 95555531;
a(4) = 13, 95555555555531;
a(5) = 17, 955555555555555531; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[(86*10^# - 221)/9] &]
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CROSSREFS
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Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
Sequence in context: A265704 A110267 A049698 * A310823 A074136 A310824
Adjacent sequences: A288820 A288821 A288822 * A288824 A288825 A288826
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KEYWORD
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nonn,more,hard
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AUTHOR
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Robert Price, Jun 17 2017
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EXTENSIONS
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a(27)-a(29) from Robert Price, Aug 31 2019
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STATUS
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approved
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