Number of directed loopless multigraphs on 4 nodes with n arcs.

Terms

a(0) =1a(1) =1a(2) =6a(3) =20a(4) =69a(5) =196a(6) =554a(7) =1368a(8) =3240a(9) =7110a(10) =14896a(11) =29624a(12) =56755a(13) =104468a(14) =186484a(15) =322800a(16) =544590a(17) =896259a(18) =1443834a(19) =2278640a(20) =3531647a(21) =5380064a(22) =8069672a(23) =11926928a(24) =17392983a(25) =25042836a(26) =35637168a(27) =50152068a(28) =69853604a(29) =96344440