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 | 0 |
| 3 | 6 |
| 4 | -36 |
| 5 | 210 |
| 6 | -990 |
| 7 | 2184 |
| 8 | 37128 |
| 9 | -863736 |
| 10 | 13020480 |
| 11 | -168384744 |
| 12 | 1940801544 |
| 13 | -18825129648 |
| 14 | 107706637584 |
| 15 | 1386022834944 |
| 16 | -73429347222720 |
| 17 | 2034345021802560 |
| 18 | -46869707752067520 |
| 19 | 976421492688165120 |
| 20 | -18675350766042871680 |
| 21 | 319467427583225518080 |