Meandric numbers for a river crossing up to 13 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) =28178a(13) =76539a(14) =240369a(15) =665127a(16) =2123408a(17) =5964627a(18) =19301713a(19) =54897139a(20) =179687084a(21) =516448412a(22) =1707004865a(23) =4950415081a(24) =16501638058a(25) =48228801029a(26) =161963084065a(27) =476588705579