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 | 1 |
| 5 | 4 |
| 6 | 20 |
| 7 | 102 |
| 8 | 630 |
| 9 | 4480 |
| 10 | 36288 |
| 11 | 329890 |
| 12 | 3326400 |
| 13 | 36846276 |
| 14 | 444787200 |
| 15 | 5811886080 |
| 16 | 81729648000 |
| 17 | 1230752346352 |
| 18 | 19760412672000 |
| 19 | 336967037143578 |
| 20 | 6082255020441600 |
| 21 | 115852476579840000 |
| 22 | 2322315553259520000 |
| 23 | 48869596859895986086 |