Meandric numbers for a river crossing up to 9 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) =3451a(11) =9156a(12) =28155a(13) =76491a(14) =240015a(15) =664363a(16) =2118804a(17) =5954414a(18) =19246816a(19) =54772539a(20) =179063855a(21) =515006165a(22) =1700131496a(23) =4934242781a(24) =16427135058a(25) =48050970367a(26) =161163698861a(27) =474656662565