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) =3a(3) =10a(4) =12a(5) =14a(6) =16a(7) =19a(8) =22a(9) =26a(10) =30a(11) =33a(12) =37a(13) =39a(14) =43a(15) =45a(16) =48a(17) =51a(18) =53a(19) =56a(20) =59a(21) =61a(22) =65a(23) =67a(24) =70a(25) =73a(26) =76a(27) =79a(28) =82a(29) =85