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 A119065 #7 Mar 28 2023 17:46:21
%S A119065 3,300,703,3003,300700,7777333333770,30770737700703,7707033303703300,
%T A119065 77300377037737303073770,3073770307037370707370300003,
%U A119065 33000770070773300000703773730,770037773370037377077730333003
%N A119065 Triangular numbers composed of digits {0,3,7}.
%C A119065 a(13) > 10^41. - _Tyler Busby_, Mar 28 2023
%H A119065 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr037.html">Tridigital Triangular Numbers</a>.
%F A119065 a(n) = A000217(A119066(n)). - _Tyler Busby_, Mar 28 2023
%Y A119065 Cf. A000217, A119066. See A119033 for a table of cross-references.
%K A119065 nonn,base
%O A119065 1,1
%A A119065 _Giovanni Resta_, May 10 2006