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A240090
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Number of partitions of n that have integer contraharmonic mean.
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2
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1, 2, 2, 3, 2, 6, 3, 7, 5, 8, 5, 17, 8, 21, 14, 31, 18, 49, 28, 56, 42, 90, 52, 146, 77, 189, 118, 257, 158, 370, 219, 530, 313, 724, 412, 999, 578, 1372, 809, 1837, 1094, 2515, 1472, 3387, 1948, 4584, 2656, 6145, 3527, 8114, 4665, 10784, 6225, 14196, 8150
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OFFSET
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1,2
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COMMENTS
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The contraharmonic mean of a set {x(1),..,x(k)} is defined as (x(1)^2 + ... + x(k)^2)/(x(1) + ... + x(k)); if the set is a partition of n, this mean is (x(1)^2 + ... + x(k)^2)/n, which is the square of the root mean square of the partition, discussed at A240090.
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LINKS
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EXAMPLE
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a(10) counts these 8 partitions: [10], [6,1,1,1,1], [5,5], [5,1,1,1,1,1], [4,3,2,1], [3,2,2,1,1,1], [2,2,2,2,2], [1,1,1,1,1,1,1,1,1,1]; e.g., [4,3,2,1] has contraharmonic mean (16 + 9 + 4 + 1)/10 = 3.
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MATHEMATICA
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z = 15; ColumnForm[t = Map[Select[IntegerPartitions[#], IntegerQ[RootMeanSquare[#]] &] &, Range[z]]] (* shows the partitions *)
ColumnForm[u = Map[Select[IntegerPartitions[#], IntegerQ[ContraharmonicMean[#]] &] &, Range[z]]] (* shows the partitions *)
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CROSSREFS
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KEYWORD
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nonn,easy
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AUTHOR
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STATUS
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approved
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