a(n) = 2^n - 27.

Terms

a(0) =-26a(1) =-25a(2) =-23a(3) =-19a(4) =-11a(5) =5a(6) =37a(7) =101a(8) =229a(9) =485a(10) =997a(11) =2021a(12) =4069a(13) =8165a(14) =16357a(15) =32741a(16) =65509a(17) =131045a(18) =262117a(19) =524261a(20) =1048549a(21) =2097125a(22) =4194277a(23) =8388581a(24) =16777189a(25) =33554405a(26) =67108837a(27) =134217701a(28) =268435429a(29) =536870885