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 | -3 |
| 3 | 11 |
| 4 | -50 |
| 5 | 274 |
| 6 | -1764 |
| 7 | 13068 |
| 8 | -109584 |
| 9 | 1026576 |
| 10 | -10628640 |
| 11 | 120543840 |
| 12 | -1486442880 |
| 13 | 19802759040 |
| 14 | -283465647360 |
| 15 | 4339163001600 |
| 16 | -70734282393600 |
| 17 | 1223405590579200 |
| 18 | -22376988058521600 |
| 19 | 431565146817638400 |
| 20 | -8752948036761600000 |
| 21 | 186244810780170240000 |