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 | -1 |
| 4 | 0 |
| 5 | 69 |
| 6 | -225 |
| 7 | -1653 |
| 8 | 3976 |
| 9 | 187401 |
| 10 | -965385 |
| 11 | -14516745 |
| 12 | 61266744 |
| 13 | 3032650893 |
| 14 | -21187300953 |
| 15 | -491788726653 |
| 16 | 2947006495920 |
| 17 | 166337505847057 |
| 18 | -1463633608132017 |
| 19 | -46261934493321105 |
| 20 | 358635306874354320 |