A062691 Triangular numbers that contain exactly 2 different digits.

10, 15, 21, 28, 36, 45, 78, 91, 171, 300, 595, 990, 1711, 2211, 3003, 5050, 5151, 5565, 5995, 6555, 8778, 10011, 66066, 222111, 255255, 333336, 500500, 600060, 828828, 887778, 1188111, 5656566, 22221111, 50005000, 51151555, 88877778, 2222211111, 5000050000

Offset: 1

Author

Erich Friedman, Jul 04 2001

Keywords

• base
• nonn
• changed

Comments

For n > 2, A309597(n) is a term. - Seiichi Manyama, Sep 15 2019

The other known infinite families of terms are A037156(n) for n > 1, A319170(n), and A383942(n). - David Radcliffe, Aug 25 2025

Examples

			300 is triangular and contains the digits 0 and 3.
		

Links

• David Radcliffe, Table of n, a(n) for n = 1..116 (terms 1..51 from Seiichi Manyama).

Crossrefs

Cf. A000217, A045914 (all digits the same), A213516, A213518, A309597.

Programs

Mathematica

Select[Accumulate[Range[14000]],Count[DigitCount[#],Except[0]]==2&] (* Harvey P. Dale, Nov 27 2011 *)

PARI

for(k=0, 1e5, if(#Set(digits(j=k*(k+1)/2))==2, print1(j", "))) \\ Seiichi Manyama, Sep 15 2019