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 | 1 |
| 3 | 7 |
| 4 | 52 |
| 5 | 332 |
| 6 | 2832 |
| 7 | 22704 |
| 8 | 242112 |
| 9 | 2662848 |
| 10 | 30620160 |
| 11 | 354965760 |
| 12 | 5057925120 |
| 13 | 68627036160 |
| 14 | 1054183818240 |
| 15 | 17469144806400 |
| 16 | 321351896678400 |
| 17 | 5609441772748800 |
| 18 | 112707637036646400 |
| 19 | 2192664093342105600 |
| 20 | 47745925079924736000 |
| 21 | 1065919878891012096000 |