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 A119140 #9 Mar 22 2023 13:11:39
%S A119140 1,6,66,171,666,1176,1711,17766,61776,67161,166176,617716,7767711,
%T A119140 61677171,77171676,161667171,166777716,611677776,66767167176,
%U A119140 77111161116,171676611666,617617761166,1116177116671,161776717667666676,1116166667661771771,6667177116611677771
%N A119140 Triangular numbers composed of digits {1,6,7}.
%H A119140 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr167.html">Tridigital Triangular Numbers</a>
%F A119140 a(n) = A000217(A119141(n)). - _Michel Marcus_, Mar 22 2023
%Y A119140 Cf. A000217, A053907, A119141. See A119033 for a table of cross-references.
%K A119140 nonn,base
%O A119140 1,2
%A A119140 _Giovanni Resta_, May 10 2006
%E A119140 a(24)-a(26) from _Tyler Busby_, Mar 22 2023