Number of series-parallel networks with n edges.

Terms

a(0) =1a(1) =2a(2) =3a(3) =6a(4) =12a(5) =26a(6) =59a(7) =146a(8) =368a(9) =976a(10) =2667a(11) =7482a(12) =21440a(13) =62622a(14) =185637a(15) =557680a(16) =1694256a(17) =5198142a(18) =16086486a(19) =50165218a(20) =157510504a(21) =497607008a(22) =1580800091a(23) =5047337994a(24) =16190223624a(25) =52153429218a(26) =168657986843a(27) =547389492416