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 | 4 |
| 3 | 30 |
| 4 | 240 |
| 5 | 2433 |
| 6 | 26388 |
| 7 | 315726 |
| 8 | 3958452 |
| 9 | 51863952 |
| 10 | 698988716 |
| 11 | 9637772716 |
| 12 | 135161761860 |
| 13 | 1920878419569 |
| 14 | 27583547221596 |
| 15 | 399310273694328 |
| 16 | 5817100622299116 |
| 17 | 85152975761167179 |
| 18 | 1251046169511714720 |
| 19 | 18428780031111768466 |
| 20 | 271964652432415737596 |