Number of essentially series achiral series-parallel networks with n elements.

Terms

a(0) =1a(1) =1a(2) =1a(3) =3a(4) =4a(5) =11a(6) =17a(7) =46a(8) =78a(9) =203a(10) =372a(11) =946a(12) =1830a(13) =4561a(14) =9207a(15) =22609a(16) =47166a(17) =114514a(18) =245154a(19) =590345a(20) =1289950a(21) =3087959a(22) =6858746a(23) =16352074a(24) =36800928a(25) =87502317a(26) =199036637a(27) =472483088a(28) =1084108363a(29) =2571356964