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

Terms

a(0) =1a(1) =3a(2) =21a(3) =135a(4) =873a(5) =5643a(6) =36477a(7) =235791a(8) =1524177a(9) =9852435a(10) =63687141a(11) =411680151a(12) =2661142329a(13) =17201894427a(14) =111194793549a(15) =718774444575