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 | -1800 |
| 7 | -10080 |
| 8 | -20160 |
| 9 | 665280 |
| 10 | 15120000 |
| 11 | 219542400 |
| 12 | 2335132800 |
| 13 | 11416204800 |
| 14 | -272432160000 |
| 15 | -11126129414400 |
| 16 | -252293974732800 |
| 17 | -4099297608806400 |
| 18 | -36217872365875200 |
| 19 | 593695670606438400 |
| 20 | 41572213064718336000 |
| 21 | 1335024565711828992000 |