a(n) = c(n-1) + c(n-3) where c is the sequence of numbers not in a.

Terms

a(0) =1a(1) =2a(2) =4a(3) =9a(4) =12a(5) =14a(6) =17a(7) =19a(8) =23a(9) =26a(10) =29a(11) =33a(12) =36a(13) =39a(14) =42a(15) =45a(16) =47a(17) =51a(18) =53a(19) =57a(20) =59a(21) =62a(22) =65a(23) =67a(24) =71a(25) =73a(26) =77a(27) =79a(28) =83a(29) =85