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 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | -6 |
| 4 | 26 |
| 5 | -140 |
| 6 | 894 |
| 7 | -6594 |
| 8 | 55152 |
| 9 | -515808 |
| 10 | 5334480 |
| 11 | -60453360 |
| 12 | 745035840 |
| 13 | -9921337920 |
| 14 | 141972324480 |
| 15 | -2172695011200 |
| 16 | 35410730342400 |
| 17 | -612356632473600 |
| 18 | 11198955424204800 |
| 19 | -215960417122867200 |
| 20 | 4379675205233664000 |