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 | -1 |
| 2 | 1 |
| 3 | -1 |
| 4 | 25 |
| 5 | -241 |
| 6 | 1081 |
| 7 | -3361 |
| 8 | 68881 |
| 9 | -1288225 |
| 10 | 11828881 |
| 11 | -69917761 |
| 12 | 1347298921 |
| 13 | -36402297361 |
| 14 | 533785676425 |
| 15 | -4949573821921 |
| 16 | 96936811739041 |
| 17 | -3274354780495681 |
| 18 | 67608887254849441 |
| 19 | -885780921100074625 |
| 20 | 18450482854023522361 |
| 21 | -718444927697360335921 |