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 | -1 |
| 3 | 0 |
| 4 | 0 |
| 5 | 0 |
| 6 | 180 |
| 7 | 0 |
| 8 | 10080 |
| 9 | 90720 |
| 10 | 0 |
| 11 | 0 |
| 12 | 179625600 |
| 13 | -1556755200 |
| 14 | -10897286400 |
| 15 | 326918592000 |
| 16 | -2615348736000 |
| 17 | -88921857024000 |
| 18 | 800296713216000 |
| 19 | -30411275102208000 |
| 20 | 152056375511040000 |
| 21 | 0 |
| 22 | -351250227430502400000 |
| 23 | -3231502092360622080000 |