a(n) = 5*2^(n-2) - 2 for n > 1, with a(1) = 1.

Terms

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