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 | 0 |
| 1 | 0 |
| 2 | 1 |
| 3 | 6 |
| 4 | 36 |
| 5 | 220 |
| 6 | 1410 |
| 7 | 9534 |
| 8 | 68040 |
| 9 | 511704 |
| 10 | 4046310 |
| 11 | 33560010 |
| 12 | 291244668 |
| 13 | 2638581972 |
| 14 | 24901833866 |
| 15 | 244333004790 |
| 16 | 2487900487440 |
| 17 | 26245651191600 |
| 18 | 286408960814862 |
| 19 | 3228529392965250 |
| 20 | 37544229610105220 |
| 21 | 449858650676764140 |