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 | 0 |
| 2 | 2 |
| 3 | -4 |
| 4 | 20 |
| 5 | -100 |
| 6 | 620 |
| 7 | -4420 |
| 8 | 35900 |
| 9 | -326980 |
| 10 | 3301820 |
| 11 | -36614980 |
| 12 | 442386620 |
| 13 | -5784634180 |
| 14 | 81393657020 |
| 15 | -1226280710980 |
| 16 | 19696509177020 |
| 17 | -335990918918980 |
| 18 | 6066382786809020 |
| 19 | -115578717622022980 |
| 20 | 2317323290554617020 |
| 21 | -48773618881154822980 |