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 | 12 |
| 5 | -76 |
| 6 | 480 |
| 7 | -3284 |
| 8 | 24752 |
| 9 | -205620 |
| 10 | 1874340 |
| 11 | -18639820 |
| 12 | 201028080 |
| 13 | -2337859200 |
| 14 | 29162187600 |
| 15 | -388263812000 |
| 16 | 5492326876800 |
| 17 | -82195910554800 |
| 18 | 1296105033322800 |
| 19 | -21449073720082000 |
| 20 | 371056401062968000 |
| 21 | -6682811910274568000 |
| 22 | 124750905841882320000 |