Number of achiral series-parallel networks with n elements.

Terms

a(0) =1a(1) =2a(2) =3a(3) =7a(4) =12a(5) =29a(6) =54a(7) =130a(8) =258a(9) =616a(10) =1274a(11) =3030a(12) =6458a(13) =15287a(14) =33335a(15) =78694a(16) =174587a(17) =411469a(18) =925246a(19) =2179010a(20) =4952389a(21) =11662221a(22) =26733827a(23) =62980863a(24) =145385388a(25) =342766624a(26) =795810810a(27) =1878109984a(28) =4381423357a(29) =10352044123