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 | 0 |
| 3 | 7 |
| 4 | 80 |
| 5 | 1023 |
| 6 | 15624 |
| 7 | 279935 |
| 8 | 5764800 |
| 9 | 134217727 |
| 10 | 3486784400 |
| 11 | 99999999999 |
| 12 | 3138428376720 |
| 13 | 106993205379071 |
| 14 | 3937376385699288 |
| 15 | 155568095557812223 |
| 16 | 6568408355712890624 |
| 17 | 295147905179352825855 |
| 18 | 14063084452067724991008 |
| 19 | 708235345355337676357631 |