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 | -2 |
| 3 | 0 |
| 4 | 12 |
| 5 | -120 |
| 6 | -120 |
| 7 | 5040 |
| 8 | -38640 |
| 9 | -181440 |
| 10 | 5412960 |
| 11 | -33264000 |
| 12 | -478336320 |
| 13 | 12194582400 |
| 14 | -50871300480 |
| 15 | -2168559993600 |
| 16 | 49692144902400 |
| 17 | -59775248332800 |
| 18 | -15819216007795200 |
| 19 | 329479616206540800 |
| 20 | 1101564635255884800 |
| 21 | -174845824790757120000 |