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A280969
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Numbers k such that 7*10^k - 17 is prime.
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0
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1, 2, 3, 11, 16, 33, 35, 37, 39, 42, 64, 94, 123, 136, 308, 309, 310, 357, 965, 1109, 2178, 8916, 9471, 18330, 62127, 252209
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OFFSET
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1,2
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COMMENTS
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For k > 1, numbers k such that the digit 6 followed by k-2 occurrences of the digit 9 followed by the digits 83 is prime (see Example section).
a(27) > 3*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because 7*10^3 - 17 = 6983 is prime.
Initial terms and associated primes:
a(1) = 1, 53;
a(2) = 2, 683;
a(3) = 3, 6983;
a(4) = 11, 699999999983;
a(5) = 16, 69999999999999983; etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[7*10^# - 17] &]
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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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