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 | 4 |
| 2 | 20 |
| 3 | 168 |
| 4 | 1200 |
| 5 | 7600 |
| 6 | 43200 |
| 7 | 407680 |
| 8 | 2712832 |
| 9 | 19707408 |
| 10 | 112560000 |
| 11 | 945677920 |
| 12 | 6768230400 |
| 13 | 47530457728 |
| 14 | 271289229120 |
| 15 | 2096760960000 |
| 16 | 16569393144832 |
| 17 | 116315256993600 |
| 18 | 597938524646400 |
| 19 | 5699431359135360 |
| 20 | 38890647857280000 |
| 21 | 270061302781670400 |