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A288821
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Numbers k such that 10^k - 401 is prime.
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0
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3, 6, 9, 11, 12, 25, 47, 166, 222, 234, 352, 407, 515, 646, 1161, 2475, 4329, 7191, 12894, 16007, 25819, 67579, 88457, 117135
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OFFSET
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1,1
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COMMENTS
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For k > 2, numbers k such that k-3 occurrences of the digit 9 followed by the digits 599 is prime (see Example section).
a(25) > 2*10^5.
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LINKS
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EXAMPLE
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6 is in this sequence because 10^6 - 401 = 999599 is prime.
Initial terms and associated primes:
a(1) = 3, 599;
a(2) = 6, 999599;
a(3) = 9, 999999599;
a(4) = 11, 99999999599;
a(5) = 12, 999999999599; etc.
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MATHEMATICA
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Select[Range[3, 100000], PrimeQ[10^# - 401] &]
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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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