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.
Do[ If[ Union[ Drop[ RotateLeft[ IntegerDigits[n(n + 1)/2]], -2]] == {9}, Print[n(n + 1)/2]], {n, 14, 10^6}]
Do[ If[ Union[ Drop[ RotateLeft[ IntegerDigits[n(n + 1)/2]], -2]] == {3}, Print[n(n + 1)/2]], {n, 14, 2 10^6}]
Do[ If[ Union[ Drop[ RotateLeft[ IntegerDigits[n(n + 1)/2]], -2]] == {2}, Print[n(n + 1)/2]], {n, 14, 2 10^6}]
Do[ If[ Union[ Drop[ RotateLeft[ IntegerDigits[n(n + 1)/2]], -2]] == {5}, Print[n(n + 1)/2]], {n, 14, 2 10^6}]
Select[Accumulate[Range[0,300]],IntegerLength[#]<3||Union[Most[Rest[IntegerDigits[#]]]]=={5}&] (* Harvey P. Dale, Aug 13 2025 *)
Do[ If[ Union[ Drop[ RotateLeft[ IntegerDigits[n(n + 1)/2]], -2]] == {6}, Print[n(n + 1)/2]], {n, 14, 2 10^6}]
Do[ If[ Union[ Drop[ RotateLeft[ IntegerDigits[n(n + 1)/2]], -2]] == {6}, Print[n(n + 1)/2]], {n, 14, 2 10^6}]
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