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 | 0 |
| 3 | -6 |
| 4 | 36 |
| 5 | -180 |
| 6 | 720 |
| 7 | -7560 |
| 8 | 236880 |
| 9 | -3099600 |
| 10 | 15120000 |
| 11 | -194594400 |
| 12 | 9989179200 |
| 13 | -131935003200 |
| 14 | 337815878400 |
| 15 | -50154760656000 |
| 16 | 2018231927712000 |
| 17 | -27611162875296000 |
| 18 | 363290246871552000 |
| 19 | -12648028196067264000 |
| 20 | 521752941995725440000 |