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 A119198 #10 Mar 27 2023 07:37:08
%S A119198 3,6,36,66,666,333336,936396,963966,3966336,3996939936,99363693366,
%T A119198 63666996693966,339666999336666,633636693339936,39696699699366996,
%U A119198 999699963339963966,36936333639936696636,6933666366666339963969663666,393996396333933393393669696696,399693363966633933999339639336
%N A119198 Triangular numbers composed of digits {3,6,9}.
%H A119198 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr369.html">Tridigital Triangular Numbers</a>.
%F A119198 a(n) = A000217(A119199(n)). - _Tyler Busby_, Mar 26 2023
%Y A119198 Cf. A000217, A053949, A119199. See A119033 for a table of cross-references.
%K A119198 nonn,base
%O A119198 1,1
%A A119198 _Giovanni Resta_, May 10 2006
%E A119198 a(19)-a(20) from _Tyler Busby_, Mar 26 2023