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 A119043 #13 Mar 31 2023 13:21:15
%S A119043 1,10,171,1711,1770,10011,111117701071,17170010070077710,
%T A119043 101711101707777771,701000700077010700,1701111717707771071,
%U A119043 10010107100007777700,101770070070071771070,170107111111710707070,1177170070777107111711,70071707101001717077017171
%N A119043 Triangular numbers composed of digits {0,1,7}.
%H A119043 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr017.html">Tridigital Triangular Numbers</a>.
%F A119043 a(n) = A000217(A119044(n)). - _Tyler Busby_, Mar 31 2023
%t A119043 Select[FromDigits/@Tuples[{0,1,7},12],OddQ[Sqrt[8#+1]]&] (* The program generates the first 8 terms of the sequence. *) (* _Harvey P. Dale_, Sep 06 2022 *)
%Y A119043 Cf. A000217, A058420, A119044. See A119033 for a table of cross-references.
%K A119043 nonn,base
%O A119043 1,2
%A A119043 _Giovanni Resta_, May 10 2006
%E A119043 a(15)-a(16) from _Tyler Busby_, Mar 22 2023