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 A320893 Numbers with an even number of prime factors (counted with multiplicity) that can be factored into squarefree semiprimes (A320911) but cannot be factored into distinct semiprimes (A320892). 24
 1296, 7776, 10000, 12960, 18144, 19440, 21600, 27216, 28512, 33696, 36000, 38416, 42336, 42768, 44064, 46656, 48600, 49248, 50544, 50625, 59616, 60000, 66096, 73872, 75168, 77760, 80352, 89424, 95256, 95904, 98784, 100000 (list; graph; refs; listen; history; text; internal format)
 OFFSET 1,1 COMMENTS A semiprime (A001358) is a product of any two not necessarily distinct primes. LINKS Table of n, a(n) for n=1..32. MATHEMATICA sqfsemfacs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[sqfsemfacs[n/d], Min@@#>=d&]], {d, Select[Rest[Divisors[n]], And[SquareFreeQ[#], PrimeOmega[#]==2]&]}]]; 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[10000], And[EvenQ[PrimeOmega[#]], strsemfacs[#]=={}, sqfsemfacs[#]!={}]&] CROSSREFS Cf. A001055, A001358, A005117, A006881, A007717, A028260, A318871, A318953, A320655, A320656, A320891, A320892, A320894, A320911, A320912, A320913. Sequence in context: A217908 A223508 A250810 * A276282 A137488 A189991 Adjacent sequences: A320890 A320891 A320892 * A320894 A320895 A320896 KEYWORD nonn AUTHOR _Gus Wiseman_, Oct 23 2018 STATUS approved

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Last modified May 26 04:31 EDT 2024. Contains 372807 sequences. (Running on oeis4.)