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 | 0 |
| 3 | -3 |
| 4 | 12 |
| 5 | -50 |
| 6 | 240 |
| 7 | -1330 |
| 8 | 8400 |
| 9 | -59580 |
| 10 | 468000 |
| 11 | -4018300 |
| 12 | 37237200 |
| 13 | -367507400 |
| 14 | 3802780800 |
| 15 | -40373385000 |
| 16 | 423927504000 |
| 17 | -4048235126000 |
| 18 | 25093796832000 |
| 19 | 288695417426000 |
| 20 | -18721925937000000 |
| 21 | 623644389813900000 |