This is a front-end for the Online Encyclopedia of Integer Sequences, made by Christian Perfect. The idea is to provide OEIS entries in non-ancient HTML, and then to think about how they're presented visually. The source code is on GitHub.
%I A308004 #15 May 13 2019 19:12:22
%S A308004 123,101,0,20,11,1
%N A308004 a(n) = smallest nonnegative number that requires n applications of the Sisyphus function x -> A073053(x) to reach 123.
%C A308004 a(n) = index of first n in A073054.
%C A308004 a(6) is currently unknown.
%D A308004 M. E. Coppenbarger, Iterations of a modified Sisyphus function, Fib. Q., 56 (No. 2, 2018), 130-141.
%e A308004 0 -> 101 -> 123 reaches 123 in two steps, so a(2) = 0.
%e A308004 1 -> 11 -> 22 -> 202 -> 303 -> 123 reaches 123 in 5 steps, so a(5) = 1.
%t A308004 id[n_]:=IntegerDigits[n]; il[n_]:=If[n!=0,IntegerLength[n],1]
%t A308004 den[n_]:=FromDigits[{Length[Select[id[n],EvenQ]],Length[Select[id[n],OddQ]],il[n]}]; numD[n_]:=Length[FixedPointList[den,n]]-2;
%t A308004 a308004[n_]:=Module[{k=0},While[numD[k]!=n,k++];k];
%t A308004 a308004/@Range[0,5] (* _Ivan N. Ianakiev_, May 13 2019 *)
%Y A308004 Cf. A073053, A073054, A100961, A171797.
%K A308004 nonn,base,more
%O A308004 0,1
%A A308004 _N. J. A. Sloane_, May 12 2019