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 | -2 |
| 2 | 8 |
| 3 | -40 |
| 4 | 222 |
| 5 | -1316 |
| 6 | 8160 |
| 7 | -52272 |
| 8 | 343220 |
| 9 | -2297682 |
| 10 | 15623760 |
| 11 | -107611608 |
| 12 | 749209832 |
| 13 | -5264005060 |
| 14 | 37277153920 |
| 15 | -265788870480 |
| 16 | 1906489923022 |
| 17 | -13747860118724 |
| 18 | 99606357848920 |
| 19 | -724732875917064 |
| 20 | 5293303253527704 |
| 21 | -38795196044205056 |