a(n) = 48*2^n - 24.

Terms

a(0) =24a(1) =72a(2) =168a(3) =360a(4) =744a(5) =1512a(6) =3048a(7) =6120a(8) =12264a(9) =24552a(10) =49128a(11) =98280a(12) =196584a(13) =393192a(14) =786408a(15) =1572840a(16) =3145704a(17) =6291432a(18) =12582888a(19) =25165800a(20) =50331624a(21) =100663272a(22) =201326568a(23) =402653160a(24) =805306344a(25) =1610612712a(26) =3221225448a(27) =6442450920a(28) =12884901864a(29) =25769803752