Meandric numbers for a river crossing up to 18 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) =9158a(12) =28178a(13) =76539a(14) =240370a(15) =665129a(16) =2123439a(17) =5964691a(18) =19302316a(19) =54898416a(20) =179696557a(21) =516468904a(22) =1707136753a(23) =4950705596a(24) =16503341064a(25) =48232611611a(26) =161984008143a(27) =476636172048