a(n) = (6^n - (-2)^n)/8.

Terms

a(0) =0a(1) =1a(2) =4a(3) =28a(4) =160a(5) =976a(6) =5824a(7) =35008a(8) =209920a(9) =1259776a(10) =7558144a(11) =45349888a(12) =272097280a(13) =1632587776a(14) =9795518464a(15) =58773127168a(16) =352638730240