a(n) = 6^n - 4^n.

Terms

a(0) =0a(1) =2a(2) =20a(3) =152a(4) =1040a(5) =6752a(6) =42560a(7) =263552a(8) =1614080a(9) =9815552a(10) =59417600a(11) =358602752a(12) =2160005120a(13) =12993585152a(14) =78095728640a(15) =469111242752