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 | 0 |
| 3 | -6 |
| 4 | 24 |
| 5 | -120 |
| 6 | 720 |
| 7 | -5040 |
| 8 | 80640 |
| 9 | -725760 |
| 10 | 7257600 |
| 11 | -79833600 |
| 12 | 1437004800 |
| 13 | -18681062400 |
| 14 | 261534873600 |
| 15 | -5230697472000 |
| 16 | 104613949440000 |
| 17 | -1778437140480000 |
| 18 | 32011868528640000 |
| 19 | -729870602452992000 |
| 20 | 17030314057236480000 |
| 21 | -408727537373675520000 |