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
Programs
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Maple
seq(value(Sum(binomial(m*(m-1),n)/2^(m+1),m=0..infinity)),n=0..20); # Vladeta Jovovic, Aug 25 2006
Formula
a(n) = Sum_{m>=0} binomial(m*(m-1),n)/2^(m+1). Column sums of A054547.
Extensions
More terms from Max Alekseyev, Aug 23 2006