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

Terms

a(0) =0a(1) =4a(2) =32a(3) =208a(4) =1280a(5) =7744a(6) =46592a(7) =279808a(8) =1679360a(9) =10077184a(10) =60465152a(11) =362795008a(12) =2176778240a(13) =13060685824a(14) =78364147712a(15) =470184951808