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 | 5 |
| 4 | -20 |
| 5 | 136 |
| 6 | -976 |
| 7 | 8840 |
| 8 | -88640 |
| 9 | 1019776 |
| 10 | -12933376 |
| 11 | 181318400 |
| 12 | -2767631360 |
| 13 | 45821372416 |
| 14 | -816582578176 |
| 15 | 15596531778560 |
| 16 | -317710534983680 |
| 17 | 6876972491309056 |
| 18 | -157606194409701376 |
| 19 | 3812752893951672320 |
| 20 | -97090584769337753600 |
| 21 | 2596013487025352605696 |