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A345164
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Number of alternating permutations of the multiset of prime factors of n.
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41
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1, 1, 1, 0, 1, 2, 1, 0, 0, 2, 1, 1, 1, 2, 2, 0, 1, 1, 1, 1, 2, 2, 1, 0, 0, 2, 0, 1, 1, 4, 1, 0, 2, 2, 2, 2, 1, 2, 2, 0, 1, 4, 1, 1, 1, 2, 1, 0, 0, 1, 2, 1, 1, 0, 2, 0, 2, 2, 1, 4, 1, 2, 1, 0, 2, 4, 1, 1, 2, 4, 1, 1, 1, 2, 1, 1, 2, 4, 1, 0, 0, 2, 1, 4, 2, 2, 2
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OFFSET
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1,6
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COMMENTS
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First differs from A335452 at a(30) = 4, A335452(30) = 6. The anti-runs (2,3,5) and (5,3,2) are not alternating.
A sequence is alternating if it is alternately strictly increasing and strictly decreasing, starting with either. For example, the partition (3,2,2,2,1) has no alternating permutation, even though it does have the anti-run permutations (2,3,2,1,2) and (2,1,2,3,2).
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LINKS
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EXAMPLE
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The a(n) alternating permutations of prime indices for n = 180, 210, 300, 420, 900:
(12132) (1324) (13132) (12143) (121323)
(21213) (1423) (13231) (13142) (132312)
(21312) (2143) (21313) (13241) (213132)
(23121) (2314) (23131) (14132) (213231)
(31212) (2413) (31213) (14231) (231213)
(3142) (31312) (21314) (231312)
(3241) (21413) (312132)
(3412) (23141) (323121)
(4132) (24131)
(4231) (31214)
(31412)
(34121)
(41213)
(41312)
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MATHEMATICA
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wigQ[y_]:=Or[Length[y]==0, Length[Split[y]]==Length[y]&&Length[Split[Sign[Differences[y]]]]==Length[y]-1];
Table[Length[Select[Permutations[Flatten[ConstantArray@@@FactorInteger[n]]], wigQ]], {n, 30}]
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CROSSREFS
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Counting all permutations gives A008480.
Dominated by A335452 (number of separations of prime factors).
Including twins (x,x) gives A344606.
Positions of nonzero terms are A345172.
A001250 counts alternating permutations.
A003242 counts anti-run compositions.
A344604 counts alternating compositions with twins.
A344654 counts non-twin partitions w/o alternating permutation, rank: A344653.
A344740 counts twins and partitions w/ alternating permutation, rank: A344742.
A345166 counts separable partitions w/o alternating permutation, rank: A345173.
A345170 counts partitions with a alternating permutation.
Cf. A001222, A071321, A071322, A316523, A316524, A333489, A335126, A344605, A344614, A344616, A344652, A345163, A345168.
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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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