a(n) = 120*2^n - 108.

Terms

a(0) =12a(1) =132a(2) =372a(3) =852a(4) =1812a(5) =3732a(6) =7572a(7) =15252a(8) =30612a(9) =61332a(10) =122772a(11) =245652a(12) =491412a(13) =982932a(14) =1965972a(15) =3932052a(16) =7864212a(17) =15728532a(18) =31457172a(19) =62914452a(20) =125829012a(21) =251658132a(22) =503316372a(23) =1006632852a(24) =2013265812a(25) =4026531732a(26) =8053063572a(27) =16106127252a(28) =32212254612a(29) =64424509332