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 | 0 |
| 2 | 0 |
| 3 | -6 |
| 4 | 24 |
| 5 | -120 |
| 6 | 1080 |
| 7 | -10080 |
| 8 | 100800 |
| 9 | -1149120 |
| 10 | 14515200 |
| 11 | -199584000 |
| 12 | 2973801600 |
| 13 | -47740492800 |
| 14 | 820928908800 |
| 15 | -15049152518400 |
| 16 | 292919058432000 |
| 17 | -6031865968128000 |
| 18 | 130990787582054400 |
| 19 | -2991455760887193600 |
| 20 | 71659101232502784000 |
| 21 | -1796424431562528768000 |