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 A119081 #9 Mar 23 2023 16:06:08
%S A119081 55,5050,7750,70500,500500,50005000,5000050000,500000500000,
%T A119081 7700750050575,50000005000000,5000000050000000,75075057775077570,
%U A119081 500000000500000000,7550055000555057000,50000000005000000000,505750055750070750570,775077070557070757070
%N A119081 Triangular numbers composed of digits {0,5,7}.
%H A119081 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr057.html">Tridigital Triangular Numbers</a>
%F A119081 a(n) = A000217(A119082(n)). - _Michel Marcus_, Mar 23 2023
%Y A119081 Cf. A000217, A119082. See A119033 for a table of cross-references.
%K A119081 nonn,base
%O A119081 1,1
%A A119081 _Giovanni Resta_, May 10 2006
%E A119081 a(16)-a(17) from _Tyler Busby_, Mar 23 2023