A121252 Number of labeled digraphs without isolated vertices and with n arcs.

1, 2, 25, 520, 15150, 567540, 25986334, 1406214816, 87803248260, 6213408758960, 491423381371208, 42958461746823376, 4112928296979824348, 428022362222941505760, 48106759414197240877200, 5807382441785322296139392

Offset

0,2

Links

Table of n, a(n) for n=0..15.

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: A198710 A074209 A209467 * A322727 A090733 A330767

Adjacent sequences: A121249 A121250 A121251 * A121253 A121254 A121255

Keyword

easy,nonn

Author

Vladeta Jovovic, Aug 22 2006

Extensions

More terms from Max Alekseyev, Aug 23 2006

Status

approved