Meandric numbers for a river crossing up to 5 parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =23a(6) =62a(7) =153a(8) =433a(9) =1120a(10) =3281a(11) =8776a(12) =26399a(13) =72423a(14) =222496a(15) =622616a(16) =1946044a(17) =5533227a(18) =17545134a(19) =50545069a(20) =162237507a(21) =472541542a(22) =1532707268a(23) =4506042037a(24) =14748997812a(25) =43709690021a(26) =144213436887a(27) =430371626111