a(n) = (1/2)*(s(n) - 1), where s = A026139.

Terms

a(0) =0a(1) =1a(2) =3a(3) =4a(4) =7a(5) =9a(6) =10a(7) =12a(8) =13a(9) =16a(10) =19a(11) =21a(12) =22a(13) =25a(14) =27a(15) =28a(16) =30a(17) =31a(18) =34a(19) =36a(20) =37a(21) =39a(22) =40a(23) =43a(24) =46a(25) =48a(26) =49a(27) =52a(28) =55a(29) =57