Meandric numbers for a river crossing any number of 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) =54898417a(20) =179696559a(21) =516468945a(22) =1707136837a(23) =4950706599a(24) =16503343162a(25) =48232630706a(26) =161984048816a(27) =476636485050