OFFSET
1,2
COMMENTS
A semiprime (A001358) is a product of any two not necessarily distinct primes.
LINKS
Amiram Eldar, Table of n, a(n) for n = 1..10000
EXAMPLE
9000 is in the sequence and can be factored in either of two ways: (4*6*15*25) or (4*9*10*25).
MATHEMATICA
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[#]!={}]&]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Oct 23 2018
STATUS
approved