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