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 | -2 |
| 3 | -2 |
| 4 | 6 |
| 5 | 6 |
| 6 | -20 |
| 7 | -20 |
| 8 | 70 |
| 9 | 70 |
| 10 | -252 |
| 11 | -252 |
| 12 | 924 |
| 13 | 924 |
| 14 | -3432 |
| 15 | -3432 |
| 16 | 12870 |
| 17 | 12870 |
| 18 | -48620 |
| 19 | -48620 |
| 20 | 184756 |
| 21 | 184756 |
| 22 | -705432 |
| 23 | -705432 |
| 24 | 2704156 |
| 25 | 2704156 |
| 26 | -10400600 |
| 27 | -10400600 |
| 28 | 40116600 |
| 29 | 40116600 |
| 30 | -155117520 |
| 31 | -155117520 |