a(n) = ((6+sqrt(3))^n + (6-sqrt(3))^n)/2.

Terms

a(0) =1a(1) =6a(2) =39a(3) =270a(4) =1953a(5) =14526a(6) =109863a(7) =838998a(8) =6442497a(9) =49623030a(10) =382873959a(11) =2956927518a(12) =22848289569a(13) =176600866734