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 | 2 |
| 3 | -6 |
| 4 | 24 |
| 5 | -120 |
| 6 | 720 |
| 7 | -5040 |
| 8 | 40320 |
| 9 | -362880 |
| 10 | 3628800 |
| 11 | -39916800 |
| 12 | 479001600 |
| 13 | -6227020800 |
| 14 | 87178291200 |
| 15 | -1307674368000 |
| 16 | 20922789888000 |
| 17 | -355687428096000 |
| 18 | 6402373705728000 |
| 19 | -121645100408832000 |
| 20 | 2432902008176640000 |