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 | -2 |
| 3 | -4 |
| 4 | -18 |
| 5 | -84 |
| 6 | -564 |
| 7 | -3984 |
| 8 | -33504 |
| 9 | -307728 |
| 10 | -3156192 |
| 11 | -35254080 |
| 12 | -429350400 |
| 13 | -5641133760 |
| 14 | -79720588800 |
| 15 | -1204747741440 |
| 16 | -19400679325440 |
| 17 | -331599765565440 |
| 18 | -5996988417784320 |
| 19 | -114408970262922240 |
| 20 | -2296442484579686400 |
| 21 | -48379417944213196800 |