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A278695
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Numbers k such that (38*10^k - 119) / 9 is prime.
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0
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1, 2, 4, 5, 8, 14, 16, 17, 20, 22, 38, 134, 136, 152, 829, 1087, 2110, 3524, 4412, 7079, 10538, 17672, 21583, 24163, 35126, 72050
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OFFSET
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1,2
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COMMENTS
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For k > 1, numbers k such that the digit 4 followed by k-2 occurrences of the digit 2 followed by the digits 09 is prime (see Example section).
a(27) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (38*10^4 - 119) / 9 = 42209 is prime.
Initial terms and associated primes:
a(1) = 1, 29;
a(2) = 2, 409;
a(3) = 4, 42209;
a(4) = 5, 422209;
a(5) = 8, 422222209; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(38*10^# - 119) / 9] &]
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PROG
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(PARI) isok(n) = isprime((38*10^n - 119) / 9); \\ Michel Marcus, Nov 27 2016
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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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