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.
| n | a(n) |
|---|---|
| 0 | 1 |
| 1 | 1 |
| 2 | -6 |
| 3 | 42 |
| 4 | -264 |
| 5 | 0 |
| 6 | 57456 |
| 7 | -1808352 |
| 8 | 40715136 |
| 9 | -643458816 |
| 10 | 0 |
| 11 | 583285038336 |
| 12 | -32763345398784 |
| 13 | 1237080874917888 |
| 14 | -31193431756591104 |
| 15 | 0 |
| 16 | 64105508174249558016 |
| 17 | -5177532237241354518528 |
| 18 | 274167069135623993032704 |
| 19 | -9487174826303791319678976 |
| 20 | 0 |