Triangle read by rows: counts series-parallel networks by the number of series connections.

Terms

a(0) =1a(1) =1a(2) =1a(3) =1a(4) =6a(5) =1a(6) =1a(7) =25a(8) =25a(9) =1a(10) =1a(11) =90a(12) =290a(13) =90a(14) =1a(15) =1a(16) =301a(17) =2450a(18) =2450a(19) =301a(20) =1a(21) =1a(22) =966a(23) =17451a(24) =41580a(25) =17451a(26) =966a(27) =1a(28) =1a(29) =3025