a(n) = 2*6^n-4^n.

Terms

a(0) =1a(1) =8a(2) =56a(3) =368a(4) =2336a(5) =14528a(6) =89216a(7) =543488a(8) =3293696a(9) =19893248a(10) =119883776a(11) =721399808a(12) =4336787456a(13) =26054279168a(14) =156459892736a(15) =939296227328