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.
?(FreeQ[Differences[#],0]&)],{n,0,20}] (* The program generates the first 21 terms of the sequence. *) (* _Harvey P. Dale has authored 0 sequences.