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 A119044 #12 Sep 08 2022 08:45:25
%S A119044 1,4,18,58,59,141,471418,185310604,451023506,1184061400,1844511706,
%T A119044 4474395400,14266749459,18444896915,48521543066,11838218371106,
%U A119044 39270880053981
%N A119044 Numbers k such that the k-th triangular number contains only digits {0,1,7}.
%C A119044 Next term exceeds 1.4*10^16.
%H A119044 G. Resta, <a href="http://www.numbersaplenty.com/tr/tr017.html">Tridigital Triangular Numbers</a>
%t A119044 Select[Range[3 10^7], Complement[IntegerDigits[Binomial[# + 1, 2]], {0, 1, 7}] == {} &] (* _Vincenzo Librandi_, Oct 06 2015 *)
%o A119044 (Magma) [n: n in [1..2*10^7] | Set(Intseq(Binomial(n+1,2))) subset [0,1,7]]; // _Vincenzo Librandi_, Oct 06 2015
%Y A119044 Cf. A000217, A119043. See A119034 for a table of cross-references.
%K A119044 nonn,base,more
%O A119044 1,2
%A A119044 _Giovanni Resta_, May 10 2006