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A289753
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Numbers k such that 2*10^k - 43 is prime.
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0
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2, 6, 8, 13, 24, 38, 42, 47, 324, 338, 3957, 4243, 4623, 4692, 4824, 5741, 7611, 8403, 10499, 21386, 56083, 132493, 135083, 136057, 138162, 193981
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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 1 followed by k-2 occurrences of the digit 9 followed by the digits 57 is prime (see Example section).
a(27) > 2*10^5.
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LINKS
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EXAMPLE
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6 is in this sequence because 2*10^6 - 43 = 1999957 is prime.
Initial terms and primes associated:
a(1) = 2, 157;
a(2) = 6, 1999957;
a(3) = 8, 199999957;
a(4) = 13, 19999999999957;
a(5) = 24, 1999999999999999999999957; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[2*10^# - 43] &]
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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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