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 | 1 |
| 2 | -1 |
| 3 | 3 |
| 4 | 0 |
| 5 | -30 |
| 6 | 90 |
| 7 | -630 |
| 8 | 0 |
| 9 | 22680 |
| 10 | -113400 |
| 11 | 1247400 |
| 12 | 0 |
| 13 | -97297200 |
| 14 | 681080400 |
| 15 | -10216206000 |
| 16 | 0 |
| 17 | 1389404016000 |
| 18 | -12504636144000 |
| 19 | 237588086736000 |
| 20 | 0 |
| 21 | -49893498214560000 |
| 22 | 548828480360160000 |
| 23 | -12623055048283680000 |
| 24 | 0 |
| 25 | 3786916514485104000000 |