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A123554
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Triangle read by rows: T(n,k) = number of labeled loopless digraphs with n nodes and k arcs (n >= 1, 0 <= k <= n*(n-1)).
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0
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1, 1, 2, 1, 1, 6, 15, 20, 15, 6, 1, 1, 12, 66, 220, 495, 792, 924, 792, 495, 220, 66, 12, 1, 1, 20, 190, 1140, 4845, 15504, 38760, 77520, 125970, 167960, 184756, 167960, 125970, 77520, 38760, 15504, 4845, 1140, 190, 20, 1, 1, 30, 435, 4060, 27405, 142506, 593775
(list; graph; refs; listen; history; internal format)
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OFFSET
| 1,3
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COMMENTS
| Row sums = A053763
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REFERENCES
| J. L. Gross and J. Yellen, eds., Handbook of Graph Theory, CRC Press, 2004; p. 521.
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EXAMPLE
| Triangle begins:
1
1 2 1
1 6 15 20 15 6 1
1 12 66 220 495 792 924 792 495 220 66 12 1
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MATHEMATICA
| Table[CoefficientList[Series[(1+x)^(2*Binomial[n, 2]), {x, 0, 2*Binomial[n, 2]}], x], {n, 1, 6}] (* Geoffrey Critzer, Nov 12 2011 *)
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CROSSREFS
| Cf. A052283 (unlabeled analogue).
Sequence in context: A165891 A039763 A094262 * A105291 A025270 A178234
Adjacent sequences: A123551 A123552 A123553 * A123555 A123556 A123557
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KEYWORD
| nonn,tabf
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AUTHOR
| N. J. A. Sloane (njas(AT)research.att.com), Nov 15 2006
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EXTENSIONS
| More terms from Vladeta Jovovic (vladeta(AT)eunet.rs), Nov 15 2006
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