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 A119117 #13 Mar 28 2023 10:48:17
%S A119117 1,3,171,1711,1717731,17131731,1313137337331,117717311713137711,
%T A119117 773113771333731171,3773733771711731311,177177371373713177137131,
%U A119117 311733717173773173111771,3311337711173711371777131,1173713773733717717113371171
%N A119117 Triangular numbers composed of digits {1,3,7}.
%H A119117 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr137.html">Tridigital Triangular Numbers</a>.
%F A119117 a(n) = A000217(A119118(n)).
%Y A119117 Cf. A000217, A119118. See A119033 for a table of cross-references.
%K A119117 nonn,base
%O A119117 1,2
%A A119117 _Giovanni Resta_, May 10 2006
%E A119117 a(14) from _Tyler Busby_, Mar 28 2023