OFFSET
1,6
COMMENTS
A semiprime (A001358) is a product of any two prime numbers.
LINKS
FORMULA
a(n) = Sum_{d|n squarefree} A322353(n/d).
EXAMPLE
The a(n) factorizations for n = 6, 16, 30, 60, 180, 210, 240, 420:
6 5*6 4*15 4*5*9 6*35 4*6*10 2*6*35
2*3 2*15 6*10 2*6*15 10*21 2*4*5*6 3*4*35
3*10 2*5*6 2*9*10 14*15 2*3*4*10 4*5*21
2*3*5 3*4*5 3*4*15 5*6*7 4*7*15
2*3*10 3*6*10 2*3*35 5*6*14
2*3*5*6 2*5*21 6*7*10
2*7*15 2*10*21
3*5*14 2*14*15
3*7*10 2*5*6*7
2*3*5*7 3*10*14
3*4*5*7
2*3*5*14
2*3*7*10
MATHEMATICA
facs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[facs[n/d], Min@@#>=d&]], {d, Rest[Divisors[n]]}]];
Table[Length[Select[facs[n], UnsameQ@@#&&SubsetQ[{1, 2}, PrimeOmega/@#]&]], {n, 100}]
PROG
(PARI) A339839(n, u=(1+n)) = if(1==n, 1, my(s=0); fordiv(n, d, if((d>1) && (d<u) && (bigomega(d)<3), s += A339839(n/d, d))); (s)); \\ Antti Karttunen, Feb 10 2023
CROSSREFS
A008966 allows only primes.
A320732 is the non-strict version.
A339742 does not allow squares of primes.
A339840 lists the positions of zeros.
A002100 counts partitions into squarefree semiprimes.
A013929 cannot be factored into distinct primes.
A293511 are a product of distinct squarefree numbers in exactly one way.
A320663 counts non-isomorphic multiset partitions into singletons or pairs.
A339841 have exactly one factorization into primes or semiprimes.
The following count factorizations:
- A001055 into all positive integers > 1.
- A320655 into semiprimes.
- A320656 into squarefree semiprimes.
- A322353 into distinct semiprimes.
- A339839 [this sequence] into distinct primes or semiprimes.
The following count vertex-degree partitions and give their Heinz numbers:
KEYWORD
nonn
AUTHOR
Gus Wiseman, Dec 20 2020
EXTENSIONS
Data section extended up to a(105) by Antti Karttunen, Feb 10 2023
STATUS
approved