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 | 8 |
| 5 | 56 |
| 6 | 408 |
| 7 | 3228 |
| 8 | 28032 |
| 9 | 267264 |
| 10 | 2787840 |
| 11 | 31662720 |
| 12 | 389560320 |
| 13 | 5166650880 |
| 14 | 73528680960 |
| 15 | 1118124000000 |
| 16 | 18098712576000 |
| 17 | 310759981056000 |
| 18 | 5642339254272000 |
| 19 | 108022399414272000 |
| 20 | 2175029897822208000 |