Meandric numbers for a river crossing up to 7 parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =23a(6) =63a(7) =155a(8) =447a(9) =1150a(10) =3433a(11) =9118a(12) =27923a(13) =75981a(14) =237379a(15) =658380a(16) =2090519a(17) =5888530a(18) =18951534a(19) =54069881a(20) =176020213a(21) =507632647a(22) =1668910009a(23) =4857449082a(24) =16106998656a(25) =47253245257a(26) =157874165288a(27) =466367068388