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 | -1 |
| 2 | 1 |
| 3 | -2 |
| 4 | 6 |
| 5 | -30 |
| 6 | 204 |
| 7 | -1944 |
| 8 | 23340 |
| 9 | -360060 |
| 10 | 6692280 |
| 11 | -151908120 |
| 12 | 4032593040 |
| 13 | -126425330640 |
| 14 | 4537069829880 |
| 15 | -187762669281600 |
| 16 | 8751745668796200 |
| 17 | -462088269954518760 |
| 18 | 27132285729049971120 |
| 19 | -1779812144439507990000 |
| 20 | 128512316060872730854560 |