OFFSET
1,2
COMMENTS
The complement starts: 8, 16, 24, 27, 30, 32, 36, 40.
LINKS
R. E. Canfield, P. Erdős and C. Pomerance, On a Problem of Oppenheim concerning "Factorisatio Numerorum", J. Number Theory 17 (1983), 1-28.
EXAMPLE
Factorizations of n = 1, 4, 12, 72:
() (4) (12) (72)
(2*2) (2*6) (8*9)
(3*4) (2*36)
(2*2*3) (3*24)
(4*18)
(6*12)
(2*4*9)
(2*6*6)
(3*3*8)
(3*4*6)
(2*2*18)
(2*3*12)
(2*2*2*9)
(2*2*3*6)
(2*3*3*4)
(2*2*2*3*3)
MATHEMATICA
facs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[facs[n/d], Min@@#>=d&]], {d, Rest[Divisors[n]]}]];
Select[Range[100], IntegerQ[Log[2, Length[facs[#]]]]&]
CROSSREFS
The same for strict integer partitions is A331022.
The least number with exactly n factorizations is A330973(n).
The least number with exactly 2^n factorizations is A330989(n).
Numbers whose inverse prime shadow belongs to this sequence are A330990.
Numbers with a prime number of factorizations are A330991.
KEYWORD
nonn
AUTHOR
Gus Wiseman, Jan 07 2020
STATUS
approved