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).

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

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: A250810 A378900 A378768 * A276282 A137488 A189991

Adjacent sequences: A320890 A320891 A320892 * A320894 A320895 A320896

Keyword

nonn

Author

Gus Wiseman, Oct 23 2018

Status

approved