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A282571
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Numbers k such that 2*10^k - 27 is prime.
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0
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2, 3, 4, 9, 26, 40, 41, 56, 82, 114, 150, 155, 203, 213, 578, 2638, 4761, 10202, 14211, 16421, 18047, 30701, 53834, 66826, 115784, 141407
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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 73 is prime (see Example section).
a(27) > 2*10^5.
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LINKS
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EXAMPLE
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3 is in this sequence because 2*10^3 - 27 = 1973 is prime.
Initial terms and primes associated:
a(1) = 2, 173;
a(2) = 3, 1973;
a(3) = 4, 19973;
a(4) = 9, 1999999973;
a(5) = 26, 199999999999999999999999973; etc.
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MATHEMATICA
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Select[Range[2, 100000], PrimeQ[2*10^# - 27] &]
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PROG
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(PARI) isok(k) = isprime(2*10^k - 27); \\ Michel Marcus, Feb 19 2017
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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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