Meandric numbers for a river crossing up to 11 parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =23a(6) =63a(7) =155a(8) =448a(9) =1152a(10) =3452a(11) =9158a(12) =28177a(13) =76537a(14) =240343a(15) =665073a(16) =2122968a(17) =5963685a(18) =19295541a(19) =54883588a(20) =179608831a(21) =516272903a(22) =1706070421a(23) =4948280698a(24) =16490889347a(25) =48203859456a(26) =161842379157a(27) =476304741819