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 | 0 |
| 3 | 0 |
| 4 | 2 |
| 5 | 2 |
| 6 | 4 |
| 7 | 28 |
| 8 | 0 |
| 9 | 20 |
| 10 | 52 |
| 11 | 280 |
| 12 | 1192 |
| 13 | 5520 |
| 14 | 20196 |
| 15 | 115936 |
| 16 | 701836 |
| 17 | 4174032 |
| 18 | 27261284 |
| 19 | 193428616 |
| 20 | 1445733328 |
| 21 | 11133210948 |