a(n) = greatest k such that s(k) = n, where s = A026366.

Terms

a(0) =3a(1) =6a(2) =11a(3) =14a(4) =19a(5) =22a(6) =25a(7) =28a(8) =33a(9) =36a(10) =41a(11) =44a(12) =47a(13) =50a(14) =55a(15) =58a(16) =63a(17) =66a(18) =71a(19) =74a(20) =79a(21) =82a(22) =85a(23) =88a(24) =93a(25) =96a(26) =101a(27) =104a(28) =107a(29) =110