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

Terms

a(0) =1a(1) =10a(2) =68a(3) =424a(4) =2576a(5) =15520a(6) =93248a(7) =559744a(8) =3358976a(9) =20154880a(10) =120931328a(11) =725592064a(12) =4353560576a(13) =26121379840a(14) =156728311808a(15) =940369936384