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A287684
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Numbers k such that 9*10^k - 23 is prime.
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0
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1, 2, 4, 13, 16, 19, 25, 44, 55, 64, 69, 183, 241, 290, 345, 398, 410, 423, 860, 1287, 2398, 2553, 3109
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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 8 followed by k-2 occurrences of the digit 9 followed by the digits 77 is prime (see Example section).
a(24) > 2*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because 9*10^4 - 23 = 89977 is prime.
Initial terms and primes associated:
a(1) = 1, 67;
a(2) = 2, 877;
a(3) = 4, 89977;
a(4) = 13, 89999999999977;
a(5) = 16, 89999999999999977; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[9*10^# - 23] &]
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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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