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A121252
Number of labeled digraphs without isolated vertices and with n arcs.
4
1, 2, 25, 520, 15150, 567540, 25986334, 1406214816, 87803248260, 6213408758960, 491423381371208, 42958461746823376, 4112928296979824348, 428022362222941505760, 48106759414197240877200, 5807382441785322296139392
OFFSET
0,2
FORMULA
a(n) = Sum_{m>=0} binomial(m*(m-1),n)/2^(m+1). Column sums of A054547.
MAPLE
seq(value(Sum(binomial(m*(m-1), n)/2^(m+1), m=0..infinity)), n=0..20); # Vladeta Jovovic, Aug 25 2006
CROSSREFS
Cf. A054545, A053418 (unlabeled).
Sequence in context: A074209 A209467 A374877 * A322727 A090733 A330767
KEYWORD
easy,nonn
AUTHOR
Vladeta Jovovic, Aug 22 2006
EXTENSIONS
More terms from Max Alekseyev, Aug 23 2006
STATUS
approved