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 | 12 |
| 5 | 40 |
| 6 | -180 |
| 7 | -1512 |
| 8 | -11760 |
| 9 | 142560 |
| 10 | 2701440 |
| 11 | 37033920 |
| 12 | -47472480 |
| 13 | -7299227520 |
| 14 | -181704466944 |
| 15 | -904179830400 |
| 16 | 40024286265600 |
| 17 | 1774386897454080 |
| 18 | 24426730612869120 |
| 19 | -217650777809310720 |
| 20 | -26326923875473536000 |
| 21 | -662608157128469637120 |