Meandric numbers for a river crossing up to 4 parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =22a(6) =61a(7) =142a(8) =420a(9) =1017a(10) =3146a(11) =7844a(12) =25083a(13) =63974a(14) =209875a(15) =545060a(16) =1824949a(17) =4810138a(18) =16374993a(19) =43695535a(20) =150820899a(21) =406669871a(22) =1420155120a(23) =3863613980a(24) =13627843933a(25) =37363313071a(26) =132933980698a(27) =366939582498