The binary expansion of a(n) is the second through n-th terms of A000002 - 1.

Terms

a(0) =0a(1) =1a(2) =3a(3) =6a(4) =12a(5) =25a(6) =50a(7) =101a(8) =203a(9) =406a(10) =813a(11) =1627a(12) =3254a(13) =6508a(14) =13017a(15) =26034a(16) =52068a(17) =104137a(18) =208275a(19) =416550a(20) =833101a(21) =1666202a(22) =3332404a(23) =6664809a(24) =13329618a(25) =26659237a(26) =53318475a(27) =106636950a(28) =213273900a(29) =426547801