a(n) gives position of first n in A057561.

Terms

a(0) =1a(1) =2a(2) =4a(3) =5a(4) =6a(5) =8a(6) =9a(7) =11a(8) =13a(9) =14a(10) =15a(11) =17a(12) =18a(13) =20a(14) =22a(15) =23a(16) =24a(17) =26a(18) =28a(19) =29a(20) =30a(21) =32a(22) =34a(23) =35a(24) =36a(25) =38a(26) =40a(27) =41a(28) =42a(29) =43