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A281113
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Number of twice-factorizations of n. Number of ways to choose a postpositive factorization of each part of a postpositive factorization of n.
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90
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1, 1, 3, 1, 3, 1, 6, 3, 3, 1, 9, 1, 3, 3, 15, 1, 9, 1, 9, 3, 3, 1, 23, 3, 3, 6, 9, 1, 12, 1, 28, 3, 3, 3, 32, 1, 3, 3, 23, 1, 12, 1, 9, 9, 3, 1, 58, 3, 9, 3, 9, 1, 23, 3, 23, 3, 3, 1, 41, 1, 3, 9, 66, 3, 12, 1, 9, 3, 12, 1, 84, 1, 3, 9, 9, 3, 12, 1, 58, 15, 3
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OFFSET
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2,3
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COMMENTS
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A postpositive number is a positive integer other than 1. A postpositive factorization of n is a finite orderless sequence of postpositive numbers whose product is n.
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LINKS
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EXAMPLE
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The a(20)=9 twice-factorizations are: ((20)), ((2*10)), ((4*5)), ((2*2*5)), ((2)*(10)), ((2)*(2*5)), ((4)*(5)), ((2*2)*(5)), ((2)*(2)*(5)).
Twice-factorizations of 32 organized by composite:
((2)(2)(2)(2)(2)) ((2)(2)(2)(2 2)) ((2)(2)(2 2 2)) ((2)(2 2)(2 2)) ((2)(2 2 2 2)) ((2 2)(2 2 2)) ((2 2 2 2 2))
((2)(2)(2)(4)) ((2)(2)(2 4)) ((2)(2 2)(4)) ((2)(4)(2 2)) ((2)(2 2 4)) ((2 2)(2 4)) ((4)(2 2 2)) ((2 2 2 4))
((2)(2)(8)) ((2)(2 8)) ((2 2)(8)) ((2 2 8))
((2)(4)(4)) ((2)(4 4)) ((4)(2 4)) ((2 4 4))
((2)(16)) ((2 16))
((4)(8)) ((4 8))
((32)).
Twice-factorizations of 32 organized by domain:
((2)(2)(2)(2)(2))
((2)(2)(2)(2 2)) ((2)(2)(2)(4))
((2)(2)(2 2 2)) ((2)(2)(2 4)) ((2)(2)(8))
((2)(2 2)(2 2)) ((2)(2 2)(4)) ((2)(4)(2 2)) ((2)(4)(4))
((2)(2 2 2 2)) ((2)(2 2 4)) ((2)(2 8)) ((2)(4 4)) ((2)(16))
((2 2)(2 2 2)) ((2 2)(2 4)) ((2 2)(8)) ((4)(2 2 2)) ((4)(2 4)) ((4)(8))
((2 2 2 2 2)) ((2 2 2 4)) ((2 2 8)) ((2 4 4)) ((2 16)) ((4 8)) ((32)).
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MATHEMATICA
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postfacs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[postfacs[n/d], Min@@#>=d&]], {d, Rest[Divisors[n]]}]];
twicefacs[n_]:=Join@@Tuples/@Map[postfacs, postfacs[n], {2}];
Table[Length[twicefacs[n]], {n, 2, 24}]
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CROSSREFS
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KEYWORD
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nonn
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AUTHOR
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STATUS
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approved
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