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 | -4 |
| 2 | 4 |
| 3 | 16 |
| 4 | 64 |
| 5 | 304 |
| 6 | 1712 |
| 7 | 11232 |
| 8 | 84384 |
| 9 | 715392 |
| 10 | 6761088 |
| 11 | 70513920 |
| 12 | 804683520 |
| 13 | 9975536640 |
| 14 | 133513989120 |
| 15 | 1919012014080 |
| 16 | 29482606540800 |
| 17 | 482183099596800 |
| 18 | 8364495012249600 |
| 19 | 153406409645260800 |
| 20 | 2965940772905779200 |
| 21 | 60291976261386240000 |