Meandric numbers for a river crossing up to 10 parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =23a(6) =63a(7) =155a(8) =448a(9) =1152a(10) =3452a(11) =9157a(12) =28176a(13) =76514a(14) =240318a(15) =664718a(16) =2122557a(17) =5959052a(18) =19289899a(19) =54828151a(20) =179538447a(21) =515641437a(22) =1705240090a(23) =4941294887a(24) =16481424379a(25) =48127922888a(26) =161736798007a(27) =475487875843