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

Terms

a(0) =1a(1) =4a(2) =14a(3) =46a(4) =146a(5) =454a(6) =1394a(7) =4246a(8) =12866a(9) =38854a(10) =117074a(11) =352246a(12) =1058786a(13) =3180454a(14) =9549554a(15) =28665046a(16) =86027906a(17) =258149254a(18) =774578834a(19) =2323998646a(20) =6972520226a(21) =20918609254a(22) =62757924914a(23) =188277969046a(24) =564842295746