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.
1,2, where M = [1, -2; 2, 1]. - _Simone Severini's wiki page.
1,2, where M = [1, -2; 2, 1]. - _Simone Severini has authored 0 sequences.