Number of semi-meanders of order n with 3 components.

Terms

a(0) =1a(1) =3a(2) =11a(3) =37a(4) =126a(5) =430a(6) =1454a(7) =4976a(8) =16880a(9) =57824a(10) =197010a(11) =675428a(12) =2310268a(13) =7927778a(14) =27205180a(15) =93448486a(16) =321537086a(17) =1105589516a(18) =3812424912a(19) =13121988240a(20) =45330375774a(21) =156172996170a(22) =540314673678