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 | 1 |
| 3 | 3 |
| 4 | 12 |
| 5 | 120 |
| 6 | 720 |
| 7 | 15120 |
| 8 | 241920 |
| 9 | 6531840 |
| 10 | 130636800 |
| 11 | 7185024000 |
| 12 | 172440576000 |
| 13 | 13450364928000 |
| 14 | 564915326976000 |
| 15 | 33894919618560000 |
| 16 | 2169274855587840000 |
| 17 | 295021380359946240000 |
| 18 | 15931154539437096960000 |
| 19 | 2724227426243743580160000 |
| 20 | 217938194099499486412800000 |