a(n) = -1/3 * (u^n-1)*(v^n-1) with u = 1+sqrt(3), v = 1-sqrt(3).

Terms

a(0) =1a(1) =1a(2) =9a(3) =13a(4) =61a(5) =117a(6) =421a(7) =949a(8) =2997a(9) =7381a(10) =21781a(11) =56277a(12) =160213a(13) =424789a(14) =1186389a(15) =3189589a(16) =8817493a(17) =23883093a(18) =65663317a(19) =178568533a(20) =489512277a(21) =1334064469a(22) =3651347797a(23) =9962435925a(24) =27244344661a(25) =74380006741a(26) =203315811669a(27) =555257419093