a(n) is smallest number which is uniquely a(j)+a(k), j<k.

Terms

a(0) =2a(1) =7a(2) =9a(3) =11a(4) =13a(5) =15a(6) =16a(7) =17a(8) =19a(9) =21a(10) =25a(11) =29a(12) =33a(13) =37a(14) =39a(15) =45a(16) =47a(17) =53a(18) =61a(19) =69a(20) =71a(21) =73a(22) =75a(23) =85a(24) =89a(25) =101a(26) =103a(27) =117a(28) =133a(29) =135