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) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | 0 |
| 4 | 4 |
| 5 | 20 |
| 6 | 144 |
| 7 | 630 |
| 8 | 5696 |
| 9 | 39366 |
| 10 | 366400 |
| 11 | 3634950 |
| 12 | 39066624 |
| 13 | 479163828 |
| 14 | 6226534272 |
| 15 | 87238701000 |
| 16 | 1315130458112 |
| 17 | 20922813624336 |
| 18 | 355586138666496 |
| 19 | 6402373841107998 |
| 20 | 121479626301440000 |