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 | 2 |
| 4 | -12 |
| 5 | 44 |
| 6 | -216 |
| 7 | 944 |
| 8 | -5296 |
| 9 | 27632 |
| 10 | -168128 |
| 11 | 975072 |
| 12 | -5391680 |
| 13 | 27882048 |
| 14 | 38412672 |
| 15 | -1986004224 |
| 16 | 61881368832 |
| 17 | -850359527680 |
| 18 | 16576603568128 |
| 19 | -222801125934592 |
| 20 | 4120672559178752 |
| 21 | -57863671042155520 |