Irreducible bridging trails on square lattice.

Terms

a(0) =1a(1) =1a(2) =2a(3) =2a(4) =2a(5) =4a(6) =18a(7) =48a(8) =96a(9) =194a(10) =398a(11) =992a(12) =2614a(13) =6496a(14) =15512a(15) =37536a(16) =92366a(17) =231544a(18) =583442a(19) =1464452a(20) =3680362a(21) =9310622