Related to self-avoiding walks on square lattice.

Terms

a(0) =1a(1) =6a(2) =29a(3) =108a(4) =393a(5) =1298a(6) =4271a(7) =13312a(8) =41469a(9) =125042a(10) =376747a(11) =1111144a(12) =3274475a(13) =9505054a(14) =27573041a(15) =79086964a(16) =226727667a(17) =644301026a(18) =1830188555a(19) =5162408200a(20) =14556754485a(21) =40811281170