Related to self-avoiding walks on square lattice.

Terms

a(0) =1a(1) =6a(2) =23a(3) =84a(4) =283a(5) =930a(6) =2921a(7) =9096a(8) =27507a(9) =82930a(10) =244819a(11) =722116a(12) =2096603a(13) =6087290a(14) =17458887a(15) =50090544a(16) =142317089a(17) =404543142