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 | 5 |
| 3 | -5 |
| 4 | 74 |
| 5 | -154 |
| 6 | 2484 |
| 7 | -8028 |
| 8 | 149904 |
| 9 | -663696 |
| 10 | 14257440 |
| 11 | -80627040 |
| 12 | 1965444480 |
| 13 | -13575738240 |
| 14 | 370643938560 |
| 15 | -3031488633600 |
| 16 | 91657072281600 |
| 17 | -867718162483200 |
| 18 | 28779361764249600 |
| 19 | -309920046408806400 |
| 20 | 11185850044938240000 |
| 21 | -135153868608460800000 |