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 | 1 |
| 2 | 4 |
| 3 | 18 |
| 4 | 120 |
| 5 | 840 |
| 6 | 7920 |
| 7 | 75600 |
| 8 | 887040 |
| 9 | 10886400 |
| 10 | 152409600 |
| 11 | 2235340800 |
| 12 | 36883123200 |
| 13 | 628929100800 |
| 14 | 11769069312000 |
| 15 | 230150688768000 |
| 16 | 4833164464128000 |
| 17 | 105639166144512000 |
| 18 | 2464913876705280000 |
| 19 | 59606099200327680000 |
| 20 | 1525429559126753280000 |
| 21 | 40464026199993876480000 |