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 | 2 |
| 3 | 3 |
| 4 | 20 |
| 5 | 90 |
| 6 | 714 |
| 7 | 5460 |
| 8 | 54704 |
| 9 | 580608 |
| 10 | 7214040 |
| 11 | 96932880 |
| 12 | 1452396912 |
| 13 | 23507621280 |
| 14 | 414102201408 |
| 15 | 7827185489760 |
| 16 | 158757800613120 |
| 17 | 3429996441661440 |
| 18 | 78775916315263488 |
| 19 | 1914627403408320000 |
| 20 | 49126748261368331520 |
| 21 | 1326584986873331189760 |