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

Terms

a(0) =1a(1) =4a(2) =6a(3) =7a(4) =11a(5) =14a(6) =18a(7) =21a(8) =23a(9) =27a(10) =29a(11) =32a(12) =35a(13) =37a(14) =41a(15) =44a(16) =47a(17) =50a(18) =53a(19) =56a(20) =59a(21) =63a(22) =65a(23) =69a(24) =72a(25) =75a(26) =78a(27) =81a(28) =83a(29) =87