Number of unoriented series-parallel networks with n elements.

Terms

a(0) =1a(1) =2a(2) =4a(3) =11a(4) =30a(5) =98a(6) =328a(7) =1193a(8) =4459a(9) =17287a(10) =68283a(11) =274726a(12) =1118960a(13) =4607578a(14) =19135274a(15) =80063095a(16) =337104367a(17) =1427274619a(18) =6072510001a(19) =25949049372a(20) =111319539096a(21) =479243000380