Meandric numbers for a river crossing two parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =3a(4) =8a(5) =14a(6) =43a(7) =81a(8) =272a(9) =538a(10) =1920a(11) =3926a(12) =14649a(13) =30694a(14) =118489a(15) =252939a(16) =1002994a(17) =2172830a(18) =8805410a(19) =19304190a(20) =79648888a(21) =176343390a(22) =738665040a(23) =1649008456a(24) =6996865599a(25) =15730575554a(26) =67491558466a(27) =152663683494a(28) =661370687363