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) |
|---|---|
| 1 | 1 |
| 2 | -1 |
| 3 | 1 |
| 4 | 0 |
| 5 | -6 |
| 6 | 30 |
| 7 | -90 |
| 8 | 0 |
| 9 | 2520 |
| 10 | -22680 |
| 11 | 113400 |
| 12 | 0 |
| 13 | -7484400 |
| 14 | 97297200 |
| 15 | -681080400 |
| 16 | 0 |
| 17 | 81729648000 |
| 18 | -1389404016000 |
| 19 | 12504636144000 |
| 20 | 0 |
| 21 | -2375880867360000 |
| 22 | 49893498214560000 |
| 23 | -548828480360160000 |