Meandric numbers for a river crossing up to 12 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) =76538a(14) =240368a(15) =665100a(16) =2123379a(17) =5964156a(18) =19301178a(19) =54890366a(20) =179679030a(21) =516360755a(22) =1706896545a(23) =4949350203a(24) =16500278295a(25) =48216373545a(26) =161946759019a(27) =476447428528