Related to series-parallel networks.

Terms

a(0) =0a(1) =1a(2) =7a(3) =27a(4) =101a(5) =337a(6) =1151a(7) =3843a(8) =12965a(9) =43773a(10) =148529a(11) =505605a(12) =1727771a(13) =5920823a(14) =20345445a(15) =70073901a(16) =241849929a(17) =836230109a(18) =2896104951a(19) =10044664507a(20) =34884102385a(21) =121293088909a(22) =422196245641