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 | 12 |
| 4 | 60 |
| 5 | 320 |
| 6 | 1950 |
| 7 | 13692 |
| 8 | 109592 |
| 9 | 986400 |
| 10 | 9864090 |
| 11 | 108505100 |
| 12 | 1302061332 |
| 13 | 16926797472 |
| 14 | 236975164790 |
| 15 | 3554627472060 |
| 16 | 56874039553200 |
| 17 | 966858672404672 |
| 18 | 17403456103284402 |
| 19 | 330665665962403980 |
| 20 | 6613313319248079980 |
| 21 | 138879579704209680000 |