Bridging trails on square lattice.

Terms

a(0) =1a(1) =3a(2) =7a(3) =17a(4) =41a(5) =101a(6) =259a(7) =669a(8) =1731a(9) =4499a(10) =11705a(11) =30623a(12) =80443a(13) =211851a(14) =558999a(15) =1477983a(16) =3914393a(17) =10384023a(18) =27585099a(19) =73366563a(20) =195341557a(21) =520640553