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 | 1 |
| 3 | 3 |
| 4 | 9 |
| 5 | 45 |
| 6 | 225 |
| 7 | 1575 |
| 8 | 11130 |
| 9 | 100170 |
| 10 | 897750 |
| 11 | 9875250 |
| 12 | 108523800 |
| 13 | 1410809400 |
| 14 | 18332414100 |
| 15 | 274986211500 |
| 16 | 4127136413400 |
| 17 | 70161319027800 |
| 18 | 1192076391706200 |
| 19 | 22649451442417800 |
| 20 | 430247983427262000 |
| 21 | 9035207651972502000 |