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 | 3 |
| 4 | 6 |
| 5 | 20 |
| 6 | 540 |
| 7 | 4284 |
| 8 | 35700 |
| 9 | 698760 |
| 10 | 10591560 |
| 11 | 155024100 |
| 12 | 3231336240 |
| 13 | 68704908480 |
| 14 | 1451024062488 |
| 15 | 35843008924800 |
| 16 | 950925111973200 |
| 17 | 25881184257364800 |
| 18 | 765471782499860160 |
| 19 | 24210772652940214320 |
| 20 | 797021694060934204800 |
| 21 | 27752674398657594805440 |