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A350139
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Number of non-weakly alternating ordered factorizations of n.
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7
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0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 2, 0, 0, 0, 0, 0, 4, 0, 0, 0, 2, 0, 2, 0, 0, 0, 0, 0, 10, 0, 0, 0, 0, 0, 2, 0, 2, 0, 0, 0, 12, 0, 0, 0, 4, 0, 2, 0, 0, 0, 2, 0, 20, 0, 0, 0, 0, 0, 2, 0, 10, 0, 0, 0, 12, 0
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OFFSET
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1,24
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COMMENTS
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The first odd term is a(180) = 69, which has, for example, the non-weakly alternating ordered factorization 2*3*5*3*2.
An ordered factorization of n is a finite sequence of positive integers > 1 with product n. Ordered factorizations are counted by A074206.
We define a sequence to be weakly alternating if it is alternately weakly increasing and weakly decreasing, starting with either.
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LINKS
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FORMULA
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EXAMPLE
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The a(n) ordered factorizations for n = 24, 36, 48, 60:
(2*3*4) (2*3*6) (2*3*8) (2*5*6)
(4*3*2) (6*3*2) (2*4*6) (3*4*5)
(2*3*3*2) (6*4*2) (5*4*3)
(3*2*2*3) (8*3*2) (6*5*2)
(2*2*3*4) (10*3*2)
(2*3*4*2) (2*3*10)
(2*4*3*2) (2*2*3*5)
(3*2*2*4) (2*3*5*2)
(4*2*2*3) (2*5*3*2)
(4*3*2*2) (3*2*2*5)
(5*2*2*3)
(5*3*2*2)
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MATHEMATICA
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facs[n_]:=If[n<=1, {{}}, Join@@Table[Map[Prepend[#, d]&, Select[facs[n/d], Min@@#>=d&]], {d, Rest[Divisors[n]]}]];
whkQ[y_]:=And@@Table[If[EvenQ[m], y[[m]]<=y[[m+1]], y[[m]]>=y[[m+1]]], {m, 1, Length[y]-1}];
Table[Length[Select[Join@@Permutations/@facs[n], !whkQ[#]&&!whkQ[-#]&]], {n, 100}]
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CROSSREFS
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Positions of nonzero terms are A122181.
As compositions with ones allowed these are ranked by A349057.
The complement is counted by A349059.
A345164 counts alternating perms of prime factors, with twins A344606.
A345170 counts partitions with an alternating permutation.
A348379 counts factorizations w/ alternating perm, complement A348380.
Cf. A003242, A138364, A339846, A339890, A344604, A345194, A347050, A347438, A347463, A347706, A349054.
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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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