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 | 1 |
| 2 | 3 |
| 3 | 13 |
| 4 | 68 |
| 5 | 420 |
| 6 | 3000 |
| 7 | 24360 |
| 8 | 221760 |
| 9 | 2237760 |
| 10 | 24796800 |
| 11 | 299376000 |
| 12 | 3911846400 |
| 13 | 55005350400 |
| 14 | 828193766400 |
| 15 | 13294689408000 |
| 16 | 226663557120000 |
| 17 | 4090405423104000 |
| 18 | 77895546753024000 |
| 19 | 1561112121913344000 |
| 20 | 32844177110384640000 |
| 21 | 723788347432550400000 |