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 | 0 |
| 1 | 0 |
| 2 | 1 |
| 3 | 4 |
| 4 | 22 |
| 5 | 140 |
| 6 | 1020 |
| 7 | 8400 |
| 8 | 77280 |
| 9 | 786240 |
| 10 | 8769600 |
| 11 | 106444800 |
| 12 | 1397088000 |
| 13 | 19718899200 |
| 14 | 297859161600 |
| 15 | 4794806016000 |
| 16 | 81947593728000 |
| 17 | 1482030950400000 |
| 18 | 28277150533632000 |
| 19 | 567677135241216000 |
| 20 | 11961768206868480000 |
| 21 | 263969867887165440000 |