Meandric numbers for a river crossing up to 3 parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =9a(5) =21a(6) =52a(7) =131a(8) =345a(9) =915a(10) =2519a(11) =6926a(12) =19711a(13) =55674a(14) =162594a(15) =468929a(16) =1398129a(17) =4100003a(18) =12433282a(19) =36960316a(20) =113678461a(21) =341785050a(22) =1063890616a(23) =3229522688a(24) =10156518859a(25) =31085477306a(26) =98635931623a(27) =304048850048a(28) =972323924567