a(n) is smallest number != a(j) + a(k), j < k and a(1) = 1, a(2) = 4.

Terms

a(0) =1a(1) =4a(2) =6a(3) =8a(4) =11a(5) =13a(6) =16a(7) =18a(8) =23a(9) =25a(10) =28a(11) =30a(12) =35a(13) =37a(14) =40a(15) =42a(16) =47a(17) =49a(18) =52a(19) =54a(20) =59a(21) =61a(22) =64a(23) =66a(24) =71a(25) =73a(26) =76a(27) =78a(28) =83a(29) =85