a(n) = 8^(n+1) - 2^(n+2).

Terms

a(0) =4a(1) =56a(2) =496a(3) =4064a(4) =32704a(5) =262016a(6) =2096896a(7) =16776704a(8) =134216704a(9) =1073739776a(10) =8589930496a(11) =68719468544a(12) =549755797504