a(n) = ((3^(n-1) - 1)^2)/4.

Terms

a(0) =0a(1) =1a(2) =16a(3) =169a(4) =1600a(5) =14641a(6) =132496a(7) =1194649a(8) =10758400a(9) =96845281a(10) =871666576a(11) =7845176329a(12) =70607118400a(13) =635465659921