a(0)=0, a(n) = 2*(a(n-1) + ceiling(n/2)) - 1 for n>0.

Terms

a(0) =0a(1) =1a(2) =3a(3) =9a(4) =21a(5) =47a(6) =99a(7) =205a(8) =417a(9) =843a(10) =1695a(11) =3401a(12) =6813a(13) =13639a(14) =27291a(15) =54597a(16) =109209a(17) =218435a(18) =436887a(19) =873793a(20) =1747605a(21) =3495231a(22) =6990483a(23) =13980989a(24) =27962001a(25) =55924027a(26) =111848079a(27) =223696185a(28) =447392397a(29) =894784823