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 A062691 #32 Aug 25 2025 11:24:23 %S A062691 10,15,21,28,36,45,78,91,171,300,595,990,1711,2211,3003,5050,5151, %T A062691 5565,5995,6555,8778,10011,66066,222111,255255,333336,500500,600060, %U A062691 828828,887778,1188111,5656566,22221111,50005000,51151555,88877778,2222211111,5000050000 %N A062691 Triangular numbers that contain exactly 2 different digits. %C A062691 For n > 2, A309597(n) is a term. - _Seiichi Manyama_, Sep 15 2019 %C A062691 The other known infinite families of terms are A037156(n) for n > 1, A319170(n), and A383942(n). - _David Radcliffe_, Aug 25 2025 %H A062691 David Radcliffe, <a href="/A062691/b062691.txt">Table of n, a(n) for n = 1..116</a> (terms 1..51 from Seiichi Manyama). %e A062691 300 is triangular and contains the digits 0 and 3. %t A062691 Select[Accumulate[Range[14000]],Count[DigitCount[#],Except[0]]==2&] (* _Harvey P. Dale_, Nov 27 2011 *) %o A062691 (PARI) for(k=0, 1e5, if(#Set(digits(j=k*(k+1)/2))==2, print1(j", "))) \\ _Seiichi Manyama_, Sep 15 2019 %Y A062691 Cf. A000217, A045914 (all digits the same), A213516, A213518, A309597. %K A062691 base,nonn,changed %O A062691 1,1 %A A062691 _Erich Friedman_, Jul 04 2001