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 | 2 |
| 3 | 6 |
| 4 | 24 |
| 5 | 128 |
| 6 | 928 |
| 7 | 9280 |
| 8 | 129152 |
| 9 | 2515200 |
| 10 | 68780544 |
| 11 | 2647000064 |
| 12 | 143580989440 |
| 13 | 10988411686912 |
| 14 | 1187350176604160 |
| 15 | 181232621966082048 |
| 16 | 39089521693818912768 |
| 17 | 11916533065969825808384 |
| 18 | 5135497592471003032846336 |
| 19 | 3128995097443083790244380672 |
| 20 | 2695613904312277811648715554816 |