Number of oriented series-parallel networks with n elements and without unit elements in parallel.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =4a(5) =9a(6) =19a(7) =43a(8) =99a(9) =235a(10) =562a(11) =1370a(12) =3369a(13) =8380a(14) =21000a(15) =53038a(16) =134759a(17) =344390a(18) =884376a(19) =2281274a(20) =5907791a(21) =15354795a(22) =40037979a(23) =104712010a(24) =274600650a(25) =721931534a(26) =1902362100a(27) =5023654075a(28) =13292543205a(29) =35237009037