a(n) = least k such that s(k) = n, where s = A026338.

Terms

a(0) =1a(1) =3a(2) =4a(3) =6a(4) =8a(5) =9a(6) =11a(7) =12a(8) =15a(9) =16a(10) =18a(11) =20a(12) =21a(13) =22a(14) =24a(15) =26a(16) =27a(17) =29a(18) =30a(19) =33a(20) =35a(21) =36a(22) =38a(23) =39a(24) =42a(25) =43a(26) =45a(27) =47a(28) =48a(29) =49