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 | 12 |
| 5 | 120 |
| 6 | 1210 |
| 7 | 13020 |
| 8 | 152880 |
| 9 | 1975960 |
| 10 | 28148400 |
| 11 | 440470800 |
| 12 | 7525441000 |
| 13 | 139375236000 |
| 14 | 2778421245600 |
| 15 | 59239029249400 |
| 16 | 1343609515248000 |
| 17 | 32274288638592000 |
| 18 | 818014942318974400 |
| 19 | 21809788600885084800 |
| 20 | 610079100418595808000 |
| 21 | 17863467401461938256000 |