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A320912
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Numbers with an even number of prime factors (counted with multiplicity) that can be factored into distinct semiprimes.
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28
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1, 4, 6, 9, 10, 14, 15, 21, 22, 24, 25, 26, 33, 34, 35, 36, 38, 39, 40, 46, 49, 51, 54, 55, 56, 57, 58, 60, 62, 65, 69, 74, 77, 82, 84, 85, 86, 87, 88, 90, 91, 93, 94, 95, 100, 104, 106, 111, 115, 118, 119, 121, 122, 123, 126, 129, 132, 133, 134, 135, 136, 140
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refs;
listen;
history;
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internal format)
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OFFSET
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1,2
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COMMENTS
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A semiprime (A001358) is a product of any two not necessarily distinct primes.
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LINKS
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Amiram Eldar, Table of n, a(n) for n = 1..10000
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EXAMPLE
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9000 is in the sequence and can be factored in either of two ways: (4*6*15*25) or (4*9*10*25).
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MATHEMATICA
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strsemfacs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[strsemfacs[n/d], Min@@#>d&]], {d, Select[Rest[Divisors[n]], PrimeOmega[#]==2&]}]];
Select[Range[100], And[EvenQ[PrimeOmega[#]], strsemfacs[#]!={}]&]
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CROSSREFS
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Cf. A001055, A001222, A001358, A007717, A028260, A320655, A320892, A320893, A320894, A320911.
Sequence in context: A219786 A302038 A063762 * A240938 A001358 A176540
Adjacent sequences: A320909 A320910 A320911 * A320913 A320914 A320915
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KEYWORD
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nonn
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AUTHOR
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Gus Wiseman, Oct 23 2018
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STATUS
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approved
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