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 | -3 |
| 2 | 0 |
| 3 | 9 |
| 4 | 45 |
| 5 | 252 |
| 6 | 1935 |
| 7 | 19989 |
| 8 | 260190 |
| 9 | 4063887 |
| 10 | 73823445 |
| 11 | 1527002694 |
| 12 | 35408499885 |
| 13 | 909389617497 |
| 14 | 25618701424680 |
| 15 | 785355764569749 |
| 16 | 26024092206299505 |
| 17 | 926859918577582332 |
| 18 | 35306305954587340515 |
| 19 | 1432301360556686816529 |
| 20 | 61649353087003554947550 |