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) |
|---|---|
| 1 | 1 |
| 2 | 1 |
| 3 | -2 |
| 4 | 9 |
| 5 | -56 |
| 6 | 420 |
| 7 | -3572 |
| 8 | 33328 |
| 9 | -334354 |
| 10 | 3559310 |
| 11 | -39838760 |
| 12 | 465743720 |
| 13 | -5658983108 |
| 14 | 71191948512 |
| 15 | -924554859776 |
| 16 | 12365546196641 |
| 17 | -169995491295312 |
| 18 | 2398380272232272 |
| 19 | -34680290150700800 |
| 20 | 513390937937217088 |
| 21 | -7773229533145403728 |