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 | -2 |
| 3 | 0 |
| 4 | 24 |
| 5 | -240 |
| 6 | 1440 |
| 7 | -5040 |
| 8 | -40320 |
| 9 | 1088640 |
| 10 | -14515200 |
| 11 | 119750400 |
| 12 | 0 |
| 13 | -24908083200 |
| 14 | 610248038400 |
| 15 | -9153720576000 |
| 16 | 62768369664000 |
| 17 | 1422749712384000 |
| 18 | -70426110763008000 |
| 19 | 1703031405723648000 |
| 20 | -24329020081766400000 |
| 21 | -51090942171709440000 |