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 A213516 #30 Apr 17 2023 06:36:08
%S A213516 0,1,3,6,10,15,21,28,36,45,55,66,78,91,171,300,595,666,990,1711,2211,
%T A213516 3003,5050,5151,5565,5995,6555,8778,10011,66066,222111,255255,333336,
%U A213516 500500,600060,828828,887778,1188111,5656566,22221111,50005000,51151555,88877778
%N A213516 Triangular numbers having only 1 or 2 different digits in base 10.
%C A213516 The list of triangular numbers containing only one digit (A045914) is finite. This list is infinite because numbers like 8888777778, 222222111111, and 500000500000 occur an infinite number of times.
%C A213516 A309597 is a subsequence. - _Seiichi Manyama_, Sep 14 2019
%H A213516 Seiichi Manyama, <a href="/A213516/b213516.txt">Table of n, a(n) for n = 1..55</a>
%t A213516 t = {}; Do[tri = n*(n+1)/2; If[Length[Union[IntegerDigits[tri]]] <= 2, AppendTo[t, tri]], {n, 0, 10^5}]; t
%t A213516 Select[Accumulate[Range[0,20000]],Count[DigitCount[#],0]>7&] (* _Harvey P. Dale_, Sep 03 2020 *)
%o A213516 (Magma) [n*(n+1)/2: n in [0..10^5] | #Set(Intseq(n*(n+1) div 2)) le 2]; // _Bruno Berselli_, Oct 27 2012
%Y A213516 Cf. A000217, A045914, A213517, A309597, A319170.
%Y A213516 Cf. A119033 (has list of sequences related to digits in triangular numbers).
%K A213516 nonn,base
%O A213516 1,3
%A A213516 _T. D. Noe_, Jun 21 2012