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 | 2 |
| 3 | 18 |
| 4 | 120 |
| 5 | 920 |
| 6 | 10320 |
| 7 | 126448 |
| 8 | 1714048 |
| 9 | 27073152 |
| 10 | 472354560 |
| 11 | 8989147904 |
| 12 | 187690331136 |
| 13 | 4245706716160 |
| 14 | 103239264593920 |
| 15 | 2691918892861440 |
| 16 | 74885151106498560 |
| 17 | 2212607133043884032 |
| 18 | 69227613551324233728 |
| 19 | 2286465386258267176960 |
| 20 | 79487593489348266557440 |