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 | 0 |
| 3 | 1 |
| 4 | 3 |
| 5 | 27 |
| 6 | 240 |
| 7 | 2985 |
| 8 | 42840 |
| 9 | 731745 |
| 10 | 14243040 |
| 11 | 313570845 |
| 12 | 7683984000 |
| 13 | 207685374435 |
| 14 | 6135743053440 |
| 15 | 196754537704725 |
| 16 | 6805907485977600 |
| 17 | 252620143716765825 |
| 18 | 10015402456976716800 |
| 19 | 422410127508300756825 |
| 20 | 18884777200534941696000 |