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 | 3 |
| 2 | 21 |
| 3 | 123 |
| 4 | 792 |
| 5 | 5292 |
| 6 | 42768 |
| 7 | 348840 |
| 8 | 3313440 |
| 9 | 33734880 |
| 10 | 382423680 |
| 11 | 4683303360 |
| 12 | 62795865600 |
| 13 | 903908678400 |
| 14 | 14018003596800 |
| 15 | 231803919244800 |
| 16 | 4081121327001600 |
| 17 | 76062618390528000 |
| 18 | 1497060883196928000 |
| 19 | 31001124301830144000 |
| 20 | 673857550403702784000 |