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 | 0 |
| 1 | 1 |
| 2 | -1 |
| 3 | 5 |
| 4 | -18 |
| 5 | 89 |
| 6 | -600 |
| 7 | 4717 |
| 8 | -42896 |
| 9 | 449073 |
| 10 | -5287680 |
| 11 | 69090581 |
| 12 | -993391872 |
| 13 | 15583801609 |
| 14 | -264816161792 |
| 15 | 4846181282685 |
| 16 | -95022445824000 |
| 17 | 1987373846425697 |
| 18 | -44163232640630784 |
| 19 | 1039121484066627877 |
| 20 | -25807915421845422080 |
| 21 | 674707915373741222841 |