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 A119166 #12 Apr 06 2023 21:41:46
%S A119166 55,2775,255255,577275,572755525755,2577527227575,727275225222727525,
%T A119166 2257227755272522252275,52722757572552577725225,
%U A119166 272527775757277272275522727772752527575
%N A119166 Triangular numbers composed of digits {2,5,7}.
%C A119166 a(11) > 10^40. - _Tyler Busby_, Mar 28 2023
%H A119166 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr257.html">Tridigital Triangular Numbers</a>.
%F A119166 a(n) = A000217(A119167(n)). - _Tyler Busby_, Mar 28 2023
%Y A119166 Cf. A000217, A030485, A119167. See A119033 for a table of cross-references.
%K A119166 nonn,base,more
%O A119166 1,1
%A A119166 _Giovanni Resta_, May 10 2006
%E A119166 a(10) from _Tyler Busby_, Mar 28 2023