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 | 3 |
| 3 | -13 |
| 4 | 73 |
| 5 | -501 |
| 6 | 4051 |
| 7 | -37633 |
| 8 | 394353 |
| 9 | -4596553 |
| 10 | 58941091 |
| 11 | -824073141 |
| 12 | 12470162233 |
| 13 | -202976401213 |
| 14 | 3535017524403 |
| 15 | -65573803186921 |
| 16 | 1290434218669921 |
| 17 | -26846616451246353 |
| 18 | 588633468315403843 |
| 19 | -13564373693588558173 |
| 20 | 327697927886085654441 |