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A256228
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Numbers k such that 4*10^k - 21 is prime.
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0
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1, 2, 4, 5, 7, 9, 10, 17, 21, 41, 51, 59, 61, 77, 79, 83, 97, 108, 427, 615, 869, 900, 966, 3150, 3239, 3932, 5218, 11941, 30558, 44697, 90334, 113874, 128343, 142810, 222253
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OFFSET
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1,2
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COMMENTS
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For k>1, numbers that begin with the digit 3 followed by k-2 occurrences of the digit 9 followed by the digits 79 are prime (see Example section).
a(36) > 3*10^5.
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LINKS
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EXAMPLE
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4 is in this sequence because 4*10^4 - 21 = 39979; is prime.
Initial terms and primes associated:
a(1) = 1, 19;
a(2) = 2, 379;
a(3) = 4, 39979;
a(4) = 5, 399979;
a(5) = 7, 39999979, etc.
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MATHEMATICA
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Select[Range[1, 100000], PrimeQ[4*10^# - 21] &]
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PROG
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CROSSREFS
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KEYWORD
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nonn,more
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AUTHOR
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EXTENSIONS
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STATUS
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approved
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