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 A119100 #21 Sep 08 2022 08:45:25
%S A119100 1,6,66,666,1686,2913,6666,9406,15761,66666,666666,6666666,47140518,
%T A119100 66666666,206951318,666666666,1512753193,6666666666,9406509886,
%U A119100 66666666666,297026066681,666666666666,6666666666666,66666666666666
%N A119100 Numbers k such that the k-th triangular number contains only digits {1,2,4}.
%C A119100 After 0, A002280 is a subsequence because A000217(A002280(m)) = (2*(10^m-1)/9)*10^m + (10^m-1)/9, which provides numbers of the type 22..2211..11 where 2's and 1's are repeated m times. - _Bruno Berselli_, Feb 24 2016
%H A119100 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr124.html">Tridigital Triangular Numbers</a>
%t A119100 Select[Range[2 10^7], Complement[IntegerDigits[# (# + 1)/2], {1, 2, 4}] == {} &] (* _Vincenzo Librandi_, Feb 24 2016 *)
%o A119100 (Magma) [n: n in [1..2*10^7] | Set(Intseq(n*(n+1) div 2)) subset [1,2,4]]; // _Vincenzo Librandi_, Feb 24 2016
%Y A119100 Cf. A000217, A002280, A119099.
%Y A119100 See A119034 for a table of cross-references.
%K A119100 nonn,base
%O A119100 1,2
%A A119100 _Giovanni Resta_, May 10 2006