Number of semi-meanders of order n with 2 components.

Terms

a(0) =1a(1) =2a(2) =6a(3) =16a(4) =48a(5) =140a(6) =428a(7) =1308a(8) =4072a(9) =12796a(10) =40432a(11) =129432a(12) =413900a(13) =1342580a(14) =4335288a(15) =14201804a(16) =46226896a(17) =152594276a(18) =500016036a(19) =1660630740a(20) =5472190206a(21) =18264517264a(22) =60475691308a(23) =202684618564a(24) =673892675030