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 | 60 |
| 6 | 490 |
| 7 | 4480 |
| 8 | 45920 |
| 9 | 524440 |
| 10 | 6619200 |
| 11 | 91568400 |
| 12 | 1377884200 |
| 13 | 22401579200 |
| 14 | 391192401600 |
| 15 | 7300174281400 |
| 16 | 144938169376000 |
| 17 | 3049711320656000 |
| 18 | 67777255079934400 |
| 19 | 1586172656920051200 |
| 20 | 38984454900431040000 |
| 21 | 1003827897443395024000 |