Meandric numbers for a river crossing up to 6 parallel roads at n points.

Terms

a(0) =1a(1) =1a(2) =2a(3) =4a(4) =10a(5) =23a(6) =63a(7) =154a(8) =446a(9) =1135a(10) =3416a(11) =8947a(12) =27712a(13) =74204a(14) =235041a(15) =640552a(16) =2065849a(17) =5711840a(18) =18696838a(19) =52321747a(20) =173413647a(21) =490279035a(22) =1642287831a(23) =4684178185a(24) =15834649146a(25) =45511151647a(26) =155077912223a(27) =448722569787