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.
k = x(x-1)...(x-k+1). Specifically, (x)^n = Sum's wiki page.
k = x(x-1)...(x-k+1). Specifically, (x)^n = Sum has authored 0 sequences.