Number of weakly connected digraphs with n arcs.

Terms

a(0) =1a(1) =1a(2) =4a(3) =12a(4) =53a(5) =237a(6) =1306a(7) =7537a(8) =47913a(9) =322253a(10) =2297874a(11) =17191216a(12) =134505656a(13) =1095715055a(14) =9267223594a(15) =81162609328a(16) =734511656413