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A280017
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Numbers k such that (10^k - 13)/3 is prime.
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0
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2, 4, 5, 8, 11, 25, 35, 270, 401, 613, 635, 768, 1283, 2941, 3409, 4266, 4391, 10744, 22979, 26766, 27743, 35514, 59174, 86906, 99239, 154494
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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 k - 2 occurrences of the digit 3 followed by the digits 29 is prime (see Example section).
a(27) > 2 * 10^5. - Price
There are no odd multiples of 3 in this sequence. If k is an odd multiple of 3, then (10^k - 13)/3 is divisible by 7. The smallest even multiple of 3 in the sequence is 270. - Alonso del Arte, Dec 31 2017
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LINKS
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EXAMPLE
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4 is in this sequence because (10^4 - 13)/3 = 3329 is prime.
Here is a listing of the initial terms and associated primes:
a(1) = 2, 29;
a(2) = 4, 3329;
a(3) = 5, 33329;
a(4) = 8, 33333329;
a(5) = 11, 33333333329; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[(10^# - 13)/3] &]
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PROG
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(PARI) isok(n) = isprime((10^n-13)/3); \\ Altug Alkan, Dec 31 2017
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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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EXTENSIONS
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STATUS
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approved
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