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) =5a(3) =8a(4) =10a(5) =15a(6) =18a(7) =21a(8) =24a(9) =26a(10) =29a(11) =31a(12) =35a(13) =37a(14) =41a(15) =43a(16) =47a(17) =50a(18) =53a(19) =57a(20) =60a(21) =63a(22) =66a(23) =69a(24) =72a(25) =75a(26) =78a(27) =81a(28) =84a(29) =87