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 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 40 |
| 5 | 264 |
| 6 | 3776 |
| 7 | 40256 |
| 8 | 1168000 |
| 9 | 34204032 |
| 10 | 1107791872 |
| 11 | 23233380352 |
| 12 | 1486675898368 |
| 13 | 38934372315136 |
| 14 | 1999103691427840 |
| 15 | 132874800979423232 |
| 16 | 20506322412604129280 |
| 17 | 776179999255323115520 |
| 18 | 107455579038104865996800 |
| 19 | 4651534843901106606571520 |
| 20 | 731092060557632280262082560 |