a(n) = c(n) + c(n-1) where c (A055562) is the sequence of numbers not in a.

Terms

a(0) =1a(1) =5a(2) =7a(3) =10a(4) =14a(5) =17a(6) =20a(7) =23a(8) =25a(9) =28a(10) =31a(11) =34a(12) =37a(13) =40a(14) =43a(15) =46a(16) =50a(17) =53a(18) =56a(19) =59a(20) =62a(21) =65a(22) =68a(23) =71a(24) =74a(25) =77a(26) =80a(27) =83a(28) =86a(29) =89