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A217837
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Numbers n such that 4n - 1 and 4n + 1 are both semiprimes.
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1
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14, 23, 30, 36, 40, 46, 51, 54, 55, 59, 62, 72, 75, 76, 80, 82, 85, 98, 103, 104, 113, 118, 129, 132, 134, 136, 138, 145, 167, 170, 172, 174, 195, 198, 204, 225, 240, 264, 270, 275, 280, 284, 285, 287, 292, 301, 314, 318, 329, 336, 337, 339, 346, 347, 348
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OFFSET
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1,1
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COMMENTS
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This is to semiprimes as A045753 is to primes.
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LINKS
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EXAMPLE
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a(1) = 14 because 56 - 1 = 5*11 and 56 + 1 = 3*19 are semiprimes, and this is the first number having this property.
a(2) = 23 because 92 - 1 = 7*13 and 92 + 1 = 3*31 are semiprimes, and this is the second number having this property.
a(3) = 30 because 120 - 1 = 7*17 and 120 + 1 = 11*11 are semiprimes, and this is the third number having this property.
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MATHEMATICA
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Select[Range[500], PrimeOmega[4# - 1] == 2 && PrimeOmega[4# + 1] == 2 &] (* Alonso del Arte, Oct 12 2012 *)
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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