Take 1, skip 2, take 3, etc.

Terms

a(0) =1a(1) =4a(2) =5a(3) =6a(4) =11a(5) =12a(6) =13a(7) =14a(8) =15a(9) =22a(10) =23a(11) =24a(12) =25a(13) =26a(14) =27a(15) =28a(16) =37a(17) =38a(18) =39a(19) =40a(20) =41a(21) =42a(22) =43a(23) =44a(24) =45a(25) =56a(26) =57a(27) =58a(28) =59a(29) =60