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 | 2 |
| 3 | 2 |
| 4 | -8 |
| 5 | -76 |
| 6 | -264 |
| 7 | 1672 |
| 8 | 36800 |
| 9 | 261648 |
| 10 | -1443680 |
| 11 | -66164704 |
| 12 | -792152448 |
| 13 | 2482671424 |
| 14 | 289529373056 |
| 15 | 5294082629760 |
| 16 | 1648955815936 |
| 17 | -2474170098704128 |
| 18 | -65494141255724544 |
| 19 | -303927676523118080 |
| 20 | 35926135133071923200 |
| 21 | 1341060635191667045376 |