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 A119218 #15 Jan 23 2025 16:45:36
%S A119218 6,55,66,666,5565,6555,5656566,5676765,6655776,56567566,575675766555,
%T A119218 677767555675,555665666566566,555667666766556,5755656755766655,
%U A119218 565767676566675765,7557657566575767666,5555555666655656666556566566566
%N A119218 Triangular numbers composed of digits {5,6,7}.
%C A119218 a(19) > 10^40. - _Tyler Busby_, Mar 28 2023
%H A119218 Giovanni Resta, <a href="http://www.numbersaplenty.com/tr/tr567.html">Tridigital Triangular Numbers</a>.
%F A119218 a(n) = A000217(A119219(n)). - _Michel Marcus_, Mar 27 2023
%t A119218 Select[Flatten[Table[FromDigits/@Tuples[{5,6,7},n],{n,10}]],OddQ[Sqrt[8#+1]]&] (* The program generates the first 10 terms of the sequence. *) (* _Harvey P. Dale_, Jan 23 2025 *)
%Y A119218 Cf. A000217, A053969, A119219. See A119033 for a table of cross-references.
%K A119218 nonn,base
%O A119218 1,1
%A A119218 _Giovanni Resta_, May 10 2006
%E A119218 a(18) from _Tyler Busby_, Mar 28 2023