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 | 4 |
| 5 | 24 |
| 6 | 138 |
| 7 | 1032 |
| 8 | 8160 |
| 9 | 75600 |
| 10 | 751680 |
| 11 | 8436960 |
| 12 | 100679040 |
| 13 | 1327052160 |
| 14 | 18525024000 |
| 15 | 280451808000 |
| 16 | 4477627123200 |
| 17 | 76690072166400 |
| 18 | 1377634946688000 |
| 19 | 26328977260185600 |
| 20 | 525869478021888000 |
| 21 | 11092929741653760000 |
| 22 | 243781091314016256000 |
| 23 | 5628622656645660672000 |