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 | 2 |
| 2 | 6 |
| 3 | 24 |
| 4 | 120 |
| 5 | 696 |
| 6 | 4536 |
| 7 | 32568 |
| 8 | 254136 |
| 9 | 2133816 |
| 10 | 19130040 |
| 11 | 182000952 |
| 12 | 1828296888 |
| 13 | 19311334200 |
| 14 | 213709376184 |
| 15 | 2470302259512 |
| 16 | 29746381049016 |
| 17 | 372270346391352 |
| 18 | 4831940144914104 |
| 19 | 64925998174811448 |
| 20 | 901626111996723384 |
| 21 | 12920858504042924856 |