Related to series-parallel networks.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =3a(5) =5a(6) =17a(7) =41a(8) =127a(9) =365a(10) =1119a(11) =3413a(12) =10685a(13) =33561a(14) =106827a(15) =342129a(16) =1104347a(17) =3584649a(18) =11701369a(19) =38374065a(20) =126395259a(21) =417908329a(22) =1386618307a(23) =4615388353a(24) =15407188529a(25) =51569669429a(26) =173033992311a(27) =581905285089