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A278591
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Numbers k such that (11*10^k - 107) / 3 is prime.
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0
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2, 3, 5, 7, 8, 11, 14, 18, 25, 39, 81, 91, 347, 391, 438, 464, 539, 818, 1051, 1125, 1598, 3384, 11966, 79867, 147313
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OFFSET
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1,1
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COMMENTS
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For k>1, numbers such that the digit 3 followed by k-2 occurrences of the digit 6 followed by the digits 31 is prime (see Example section).
a(26) > 2*10^5.
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LINKS
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Table of n, a(n) for n=1..25.
Makoto Kamada, Factorization of near-repdigit-related numbers.
Makoto Kamada, Search for 36w31.
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EXAMPLE
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3 is in this sequence because (11*10^3 - 107) / 3 = 3631 is prime.
Initial terms and primes associated:
a(1) = 2, 331;
a(2) = 3, 3631;
a(3) = 5, 366631;
a(4) = 7, 36666631;
a(5) = 8, 366666631; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(11*10^# - 107) / 3] &]
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PROG
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(PARI) is(n)=ispseudoprime((11*10^n - 107)/3) \\ Charles R Greathouse IV, Jun 13 2017
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CROSSREFS
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Cf. A056654, A268448, A269303, A270339, A270613, A270831, A270890, A270929, A271269.
Sequence in context: A186285 A190855 A190810 * A191121 A026401 A069353
Adjacent sequences: A278588 A278589 A278590 * A278592 A278593 A278594
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KEYWORD
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nonn,more,hard
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AUTHOR
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Robert Price, Nov 23 2016
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EXTENSIONS
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First comment and second link corrected by Robert Price, May 23 2018
a(25) from Robert Price, Sep 30 2018
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STATUS
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approved
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