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 A104611 #6 Jul 09 2020 14:04:42
%S A104611 1,3,6,1,1,2,3,0,3,4,1,0,1,4,2,1,6,5,1,4,4,2,5,6,2,2,4,3,2,5,3,5,2,4,
%T A104611 4,2,5,6,5,5,4,4,6,0,2,1,3,1,1,3,1,2,2,1,1,1,1,5,4,4,4,1,1,3,1,2,6,0,
%U A104611 2,2,3,0,2,1,2,2,2,2,0,6,2,2,5,6,4,3,6,0,3,6,2,2,2,3,0,3,3,5,6,6,2,4,3,1,4
%N A104611 Write the natural numbers in base 7 in a triangle with k digits in the k-th row, as shown below. Sequence gives the leading diagonal.
%C A104611 1
%C A104611 23
%C A104611 456
%C A104611 1011
%C A104611 12131
%C A104611 415162...
%t A104611 t = Flatten[ IntegerDigits[ Range[1500], 7]]; t[[Table[n(n + 1)/2, {n, 105}]]]
%t A104611 Module[{d = Flatten[Table[IntegerDigits[n, 7], {n, 2000}]]}, TakeList[ d, Range[(Sqrt[8*Length[d] + 1] - 1)/2]]][[All, -1]] (* Requires Mathematica version 11 or later *) (* _Harvey P. Dale_, Jul 09 2020 *)
%Y A104611 Cf. A104606, A104607, A104608, A104609, A104610, A104612, A104613, A091425, A104614, A104615, A104616, A104617, A104618, A104619, A104620.
%K A104611 base,nonn
%O A104611 1,2
%A A104611 _Robert G. Wilson v_, Mar 16 2005