a(n) = 5*2^n - 2.

Terms

a(0) =3a(1) =8a(2) =18a(3) =38a(4) =78a(5) =158a(6) =318a(7) =638a(8) =1278a(9) =2558a(10) =5118a(11) =10238a(12) =20478a(13) =40958a(14) =81918a(15) =163838a(16) =327678a(17) =655358a(18) =1310718a(19) =2621438a(20) =5242878a(21) =10485758a(22) =20971518a(23) =41943038a(24) =83886078a(25) =167772158a(26) =335544318a(27) =671088638a(28) =1342177278a(29) =2684354558