a(n) = 2^n - 5.

Terms

a(0) =-4a(1) =-3a(2) =-1a(3) =3a(4) =11a(5) =27a(6) =59a(7) =123a(8) =251a(9) =507a(10) =1019a(11) =2043a(12) =4091a(13) =8187a(14) =16379a(15) =32763a(16) =65531a(17) =131067a(18) =262139a(19) =524283a(20) =1048571a(21) =2097147a(22) =4194299a(23) =8388603a(24) =16777211a(25) =33554427a(26) =67108859a(27) =134217723a(28) =268435451a(29) =536870907