a(n) = 2*(3^n - 1).

Terms

a(0) =0a(1) =4a(2) =16a(3) =52a(4) =160a(5) =484a(6) =1456a(7) =4372a(8) =13120a(9) =39364a(10) =118096a(11) =354292a(12) =1062880a(13) =3188644a(14) =9565936a(15) =28697812a(16) =86093440a(17) =258280324a(18) =774840976a(19) =2324522932a(20) =6973568800a(21) =20920706404a(22) =62762119216a(23) =188286357652a(24) =564859072960