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 | 2 |
| 4 | 6 |
| 5 | 24 |
| 6 | 200 |
| 7 | 1560 |
| 8 | 12936 |
| 9 | 130368 |
| 10 | 1458432 |
| 11 | 17623440 |
| 12 | 233922480 |
| 13 | 3376625472 |
| 14 | 52382131776 |
| 15 | 870882440064 |
| 16 | 15459372915840 |
| 17 | 291596692838400 |
| 18 | 5824039155720192 |
| 19 | 122814724467223296 |
| 20 | 2726547887891407104 |
| 21 | 63562453551393223680 |
| 22 | 1552499303360183700480 |