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A293511
Numbers that can be written as a product of distinct squarefree numbers in exactly one way.
9
1, 2, 3, 5, 7, 11, 12, 13, 17, 18, 19, 20, 23, 28, 29, 31, 36, 37, 41, 43, 44, 45, 47, 50, 52, 53, 59, 61, 63, 67, 68, 71, 73, 75, 76, 79, 83, 89, 92, 97, 98, 99, 100, 101, 103, 107, 109, 113, 116, 117, 120, 124, 127, 131, 137, 139, 147, 148, 149, 151, 153
OFFSET
1,2
COMMENTS
First differs from A212166 at a(128) = 363, A212166(128) = 360.
EXAMPLE
360 is not in the sequence because it has two possible expressions: 2*3*6*10 or 2*6*30.
MATHEMATICA
nn=300;
sqfacs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[sqfacs[n/d], Min@@#>d&]], {d, Select[Rest[Divisors[n]], SquareFreeQ]}]];
Select[Range[nn], Length[sqfacs[#]]===1&]
KEYWORD
nonn
AUTHOR
Gus Wiseman, Oct 11 2017
STATUS
approved