a(n) = 2^(n-1)*(3^n-2^(n+1)+1).

Terms

a(0) =0a(1) =0a(2) =4a(3) =48a(4) =400a(5) =2880a(6) =19264a(7) =123648a(8) =774400a(9) =4776960a(10) =29185024a(11) =177205248a(12) =1071616000a(13) =6463242240a(14) =38913654784a(15) =234018766848