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A139594
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Number of different n X n symmetric matrices with natural number entries summing to 4. Also number of symmetric oriented graphs with 4 arcs on n points.
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1
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0, 1, 9, 39, 116, 275, 561, 1029, 1744, 2781, 4225, 6171, 8724, 11999, 16121, 21225, 27456, 34969, 43929, 54511, 66900, 81291, 97889, 116909, 138576, 163125, 190801, 221859, 256564, 295191, 338025, 385361, 437504, 494769, 557481, 625975, 700596
(list; graph; refs; listen; history; internal format)
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OFFSET
| 0,3
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FORMULA
| a(n) = coefficient of X^4 in 1/((1-X)^m*(1-X^2)^binomial(m,2))
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MAPLE
| dd := proc(n, m) coeftayl(1/((1-X)^m*(1-X^2)^binomial(m, 2)), X=0, n); seq(dd(4, m), m=0..N);
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CROSSREFS
| For 3 in place of 4 this gives A005900.
Sequence in context: A158447 A023163 A054121 * A034263 A060929 A124851
Adjacent sequences: A139591 A139592 A139593 * A139595 A139596 A139597
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KEYWORD
| easy,nonn
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AUTHOR
| Marc A. A. van Leeuwen (Marc.van-Leeuwen(AT)math.univ-poitiers.fr), Jun 12 2008
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