OFFSET
1,6
COMMENTS
A squarefree semiprime (A006881) is a product of any two distinct prime numbers.
Conjecture: also the number of semistandard Young tableaux whose entries are the prime indices of n (A323437).
Is this a duplicate of A323437? - R. J. Mathar, Jan 05 2021
LINKS
FORMULA
EXAMPLE
The a(n) factorizations for n = 36, 60, 180, 360, 420, 840:
6*6 6*10 5*6*6 6*6*10 2*6*35 6*10*14
2*3*6 2*5*6 2*6*15 2*5*6*6 5*6*14 2*2*6*35
2*2*3*3 2*2*15 3*6*10 2*2*6*15 6*7*10 2*5*6*14
2*3*10 2*3*5*6 2*3*6*10 2*10*21 2*6*7*10
2*2*3*5 2*2*3*15 2*2*3*5*6 2*14*15 2*2*10*21
2*3*3*10 2*2*2*3*15 2*5*6*7 2*2*14*15
2*2*3*3*5 2*2*3*3*10 3*10*14 2*2*5*6*7
2*2*2*3*3*5 2*2*3*35 2*3*10*14
2*2*5*21 2*2*2*3*35
2*2*7*15 2*2*2*5*21
2*3*5*14 2*2*2*7*15
2*3*7*10 2*2*3*5*14
2*2*3*5*7 2*2*3*7*10
2*2*2*3*5*7
MATHEMATICA
sqpe[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[sqpe[n/d], Min@@#>=d&]], {d, Select[Divisors[n], PrimeQ[#]||SquareFreeQ[#]&&PrimeOmega[#]==2&]}]];
Table[Length[sqpe[n]], {n, 100}]
CROSSREFS
KEYWORD
nonn
AUTHOR
Gus Wiseman, Dec 22 2020
STATUS
approved