Number of semi-meanders of order n with 4 components.

Terms

a(0) =1a(1) =4a(2) =17a(3) =66a(4) =254a(5) =956a(6) =3584a(7) =13256a(8) =49052a(9) =179552a(10) =658560a(11) =2394504a(12) =8724464a(13) =31575096a(14) =114451388a(15) =412811544a(16) =1490190544a(17) =5360943684a(18) =19288139802a(19) =69245171564a(20) =248463024330a(21) =890477645192