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 | 3 |
| 4 | -4 |
| 5 | -30 |
| 6 | 294 |
| 7 | 3780 |
| 8 | -7904 |
| 9 | -444528 |
| 10 | 78840 |
| 11 | 99657360 |
| 12 | 539299848 |
| 13 | -27852945120 |
| 14 | -361237078944 |
| 15 | 10124338180320 |
| 16 | 258341121976320 |
| 17 | -4020500134465920 |
| 18 | -205187357182405824 |
| 19 | 1330097523844832640 |
| 20 | 186823640933648588160 |
| 21 | 500469438126120583680 |