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 | 3 |
| 4 | 9 |
| 5 | -15 |
| 6 | 135 |
| 7 | -2205 |
| 8 | 21105 |
| 9 | 76545 |
| 10 | 694575 |
| 11 | -6392925 |
| 12 | -56600775 |
| 13 | 66891825 |
| 14 | -19964169225 |
| 15 | 741313447875 |
| 16 | 5375639894625 |
| 17 | 44667168170625 |
| 18 | -2328500019470625 |
| 19 | 5663134786183875 |
| 20 | -466442955127524375 |
| 21 | 11513119609487120625 |