a(n) = 4*(4^n-3^n).

Terms

a(0) =0a(1) =4a(2) =28a(3) =148a(4) =700a(5) =3124a(6) =13468a(7) =56788a(8) =235900a(9) =969844a(10) =3958108a(11) =16068628a(12) =64983100a(13) =262058164a(14) =1054609948a(15) =4237571668a(16) =17007682300a(17) =68202916084a(18) =273328224988