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A281828
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Numbers k such that 7*10^k - 89 is prime.
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0
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3, 4, 6, 7, 12, 13, 23, 28, 42, 51, 66, 247, 291, 623, 3429, 3892, 4416, 7366, 7991, 8737, 10151, 10209, 40129, 41604, 50818, 68088
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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 6 followed by k-2 occurrences of the digit 9 followed by the digits 11 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 7*10^4 - 89 = 69911 is prime.
Initial terms and primes associated:
a(1) = 3, 6911;
a(2) = 4, 69911;
a(3) = 6, 6999911;
a(4) = 7, 69999911;
a(5) = 12, 6999999999911; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[7*10^# - 89] &]
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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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