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A337484
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Number of ordered triples of positive integers summing to n that are neither strictly increasing nor strictly decreasing.
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12
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0, 0, 0, 1, 3, 6, 8, 13, 17, 22, 28, 35, 41, 50, 58, 67, 77, 88, 98, 111, 123, 136, 150, 165, 179, 196, 212, 229, 247, 266, 284, 305, 325, 346, 368, 391, 413, 438, 462, 487, 513, 540, 566, 595, 623, 652, 682, 713, 743, 776, 808, 841, 875, 910, 944, 981, 1017
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OFFSET
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0,5
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LINKS
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FORMULA
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G.f.: x^3*(1 + 2*x + 2*x^2 - x^3) / ((1 - x)^3*(1 + x)*(1 + x + x^2)).
a(n) = a(n-1) + a(n-2) - a(n-4) - a(n-5) + a(n-6) for n>6.
(End)
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EXAMPLE
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The a(3) = 1 through a(7) = 13 triples:
(1,1,1) (1,1,2) (1,1,3) (1,1,4) (1,1,5)
(1,2,1) (1,2,2) (1,3,2) (1,3,3)
(2,1,1) (1,3,1) (1,4,1) (1,4,2)
(2,1,2) (2,1,3) (1,5,1)
(2,2,1) (2,2,2) (2,1,4)
(3,1,1) (2,3,1) (2,2,3)
(3,1,2) (2,3,2)
(4,1,1) (2,4,1)
(3,1,3)
(3,2,2)
(3,3,1)
(4,1,2)
(5,1,1)
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MATHEMATICA
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Table[Length[Select[Join@@Permutations/@IntegerPartitions[n, {3}], !Less@@#&&!Greater@@#&]], {n, 0, 15}]
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CROSSREFS
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A242771 allows strictly increasing but not strictly decreasing triples.
A337481 counts these compositions of any length.
A001399(n - 6) counts unordered strict triples.
A218004 counts strictly increasing or weakly decreasing compositions.
A332745 counts partitions with weakly increasing or weakly decreasing run-lengths.
A332835 counts compositions with weakly increasing or weakly decreasing run-lengths.
A337483 counts triples either weakly increasing or weakly decreasing.
Cf. A000212, A000217, A001840, A014311, A046691, A128422, A156040, A332834, A337461, A337482, A337561, A337603, A337604.
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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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