a(n) is the smallest composite number with the sum of digits = the n-th composite number.

Terms

a(0) =4a(1) =6a(2) =8a(3) =9a(4) =28a(5) =39a(6) =68a(7) =69a(8) =88a(9) =99a(10) =299a(11) =399a(12) =589a(13) =699a(14) =799a(15) =899a(16) =999a(17) =2899a(18) =3999a(19) =5999a(20) =6999a(21) =7999a(22) =9899a(23) =9999a(24) =29999a(25) =39999a(26) =58999a(27) =69999a(28) =89999a(29) =99999