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 | 0 |
| 2 | 0 |
| 3 | 1 |
| 4 | 4 |
| 5 | 20 |
| 6 | 130 |
| 7 | 980 |
| 8 | 8400 |
| 9 | 80920 |
| 10 | 865200 |
| 11 | 10164000 |
| 12 | 130114600 |
| 13 | 1802600800 |
| 14 | 26867640800 |
| 15 | 428661633400 |
| 16 | 7288513232000 |
| 17 | 131558835408000 |
| 18 | 2512282795422400 |
| 19 | 50600743739145600 |
| 20 | 1071998968264224000 |
| 21 | 23829055696093648000 |
| 22 | 554524256514356128000 |