a(n) = greatest k such that s(k) = n, where s = A026358.

Terms

a(0) =1a(1) =4a(2) =7a(3) =12a(4) =15a(5) =20a(6) =23a(7) =26a(8) =29a(9) =34a(10) =37a(11) =42a(12) =45a(13) =48a(14) =51a(15) =56a(16) =59a(17) =64a(18) =67a(19) =72a(20) =75a(21) =80a(22) =83a(23) =86a(24) =89a(25) =94a(26) =97a(27) =102a(28) =105a(29) =108