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A285940
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Numbers k such that (2*10^k + 67)/3 is prime.
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0
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0, 1, 2, 4, 7, 40, 43, 74, 98, 181, 186, 428, 532, 644, 1664, 2016, 2476, 2911, 2959, 3007, 5964, 7758, 22231, 92152
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OFFSET
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1,3
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COMMENTS
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For k > 1, numbers k such that k-2 occurrences of the digit 6 followed by the digits 89 is prime (see Example section).
a(25) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because (2*10^4 + 67)/3 = 6689 is prime.
Initial terms and associated primes:
a(1) = 0, 23;
a(2) = 1, 29;
a(3) = 2, 89;
a(4) = 4, 6689;
a(5) = 7, 6666689; etc.
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MATHEMATICA
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Select[Range[0, 100000], PrimeQ[(2*10^# + 67)/3] &]
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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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