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 | -1 |
| 4 | 6 |
| 5 | -31 |
| 6 | 120 |
| 7 | -337 |
| 8 | -784 |
| 9 | 24705 |
| 10 | -288000 |
| 11 | 2451679 |
| 12 | -14032128 |
| 13 | -17936543 |
| 14 | 2173889536 |
| 15 | -42895630065 |
| 16 | 583266662400 |
| 17 | -5396647099903 |
| 18 | 5119183650816 |
| 19 | 1239561882325439 |
| 20 | -36754121131294720 |
| 21 | 708575518706816481 |