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 | 10 |
| 6 | 0 |
| 7 | 8 |
| 8 | 24 |
| 9 | 72 |
| 10 | 116 |
| 11 | 384 |
| 12 | 1660 |
| 13 | 7344 |
| 14 | 24364 |
| 15 | 130408 |
| 16 | 743360 |
| 17 | 4242704 |
| 18 | 27018788 |
| 19 | 190618152 |
| 20 | 1431986780 |
| 21 | 11053915716 |