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 | 6 |
| 3 | 20 |
| 4 | 28 |
| 5 | -936 |
| 6 | -23672 |
| 7 | -469456 |
| 8 | -9112560 |
| 9 | -182135008 |
| 10 | -3804634784 |
| 11 | -83297957568 |
| 12 | -1906560847424 |
| 13 | -45349267830400 |
| 14 | -1110454747949952 |
| 15 | -27582769902812416 |
| 16 | -677408818380914432 |
| 17 | -15581576995770441216 |
| 18 | -284593895830642711040 |