a(n) = 24*2^n - 18.

Terms

a(0) =6a(1) =30a(2) =78a(3) =174a(4) =366a(5) =750a(6) =1518a(7) =3054a(8) =6126a(9) =12270a(10) =24558a(11) =49134a(12) =98286a(13) =196590a(14) =393198a(15) =786414a(16) =1572846a(17) =3145710a(18) =6291438a(19) =12582894a(20) =25165806a(21) =50331630a(22) =100663278a(23) =201326574a(24) =402653166a(25) =805306350a(26) =1610612718a(27) =3221225454a(28) =6442450926a(29) =12884901870