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

Terms

a(0) =3a(1) =4a(2) =5a(3) =7a(4) =10a(5) =15a(6) =19a(7) =21a(8) =24a(9) =26a(10) =29a(11) =31a(12) =34a(13) =37a(14) =40a(15) =43a(16) =47a(17) =50a(18) =53a(19) =57a(20) =60a(21) =63a(22) =67a(23) =69a(24) =73a(25) =75a(26) =79a(27) =81a(28) =85a(29) =87