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 | 9 |
| 4 | 72 |
| 5 | 600 |
| 6 | 5400 |
| 7 | 52920 |
| 8 | 564480 |
| 9 | 6531840 |
| 10 | 81648000 |
| 11 | 1097712000 |
| 12 | 15807052800 |
| 13 | 242853811200 |
| 14 | 3966612249600 |
| 15 | 68652904320000 |
| 16 | 1255367393280000 |
| 17 | 24186745110528000 |
| 18 | 489781588488192000 |
| 19 | 10400656084955136000 |
| 20 | 231125690776780800000 |
| 21 | 5364548928029491200000 |