A 3-way generalization of series-parallel networks with n unlabeled edges.

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =15a(5) =36a(6) =99a(7) =270a(8) =783a(9) =2298a(10) =6936a(11) =21204a(12) =65895a(13) =206862a(14) =656253a(15) =2098602a(16) =6761028a(17) =21917364a(18) =71450229a(19) =234070806a(20) =770216253a(21) =2544458592a(22) =8435990916a(23) =28060099692a(24) =93612265143a(25) =313153860210