Meandric numbers: number of ways a river can cross a road n times.

Terms

a(0) =1a(1) =1a(2) =1a(3) =2a(4) =3a(5) =8a(6) =14a(7) =42a(8) =81a(9) =262a(10) =538a(11) =1828a(12) =3926a(13) =13820a(14) =30694a(15) =110954a(16) =252939a(17) =933458a(18) =2172830a(19) =8152860a(20) =19304190a(21) =73424650a(22) =176343390a(23) =678390116a(24) =1649008456a(25) =6405031050a(26) =15730575554a(27) =61606881612a(28) =152663683494a(29) =602188541928