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 | -12 |
| 5 | 29 |
| 6 | -165 |
| 7 | 993 |
| 8 | -7056 |
| 9 | 62649 |
| 10 | -540225 |
| 11 | 5044445 |
| 12 | -56372844 |
| 13 | 685980437 |
| 14 | -8868937077 |
| 15 | 124034330553 |
| 16 | -1855540686720 |
| 17 | 29570974192753 |
| 18 | -502444027531233 |
| 19 | 9041323571823925 |
| 20 | -171561171727515820 |
| 21 | 3427632445624110605 |