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A281642
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Numbers k such that 4*10^k - 57 is prime.
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0
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3, 7, 10, 15, 28, 35, 70, 183, 243, 298, 366, 388, 607, 791, 827, 1704, 1732, 1743, 2755, 2766, 2983, 3312, 7173, 16365, 18234, 102163, 104461, 136813, 154078
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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 3 followed by k-2 occurrences of the digit 9 followed by the digits 43 is prime (see Example section).
a(30) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because 4*10^3 - 57 = 3943 is prime.
Initial terms and primes associated:
a(1) = 3, 3943;
a(2) = 7, 39999943;
a(3) = 10, 39999999943;
a(4) = 15, 3999999999999943;
a(5) = 28, 39999999999999999999999999943; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[4*10^# - 57] &]
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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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