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 | -6 |
| 4 | 36 |
| 5 | -240 |
| 6 | 1920 |
| 7 | -17640 |
| 8 | 183120 |
| 9 | -2116800 |
| 10 | 26943840 |
| 11 | -374220000 |
| 12 | 5628934080 |
| 13 | -91122071040 |
| 14 | 1579034096640 |
| 15 | -29155689763200 |
| 16 | 571308920582400 |
| 17 | -11838533804697600 |
| 18 | 258608278645516800 |
| 19 | -5938673374272038400 |
| 20 | 143003892952893772800 |