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) |
|---|---|
| 1 | 0 |
| 2 | 2 |
| 3 | 16 |
| 4 | 114 |
| 5 | 800 |
| 6 | 5602 |
| 7 | 39216 |
| 8 | 274514 |
| 9 | 1921600 |
| 10 | 13451202 |
| 11 | 94158416 |
| 12 | 659108914 |
| 13 | 4613762400 |
| 14 | 32296336802 |
| 15 | 226074357616 |
| 16 | 1582520503314 |
| 17 | 11077643523200 |
| 18 | 77543504662402 |
| 19 | 542804532636816 |
| 20 | 3799631728457714 |
| 21 | 26597422099204000 |