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 | 0 |
| 2 | 2 |
| 3 | 18 |
| 4 | 168 |
| 5 | 1920 |
| 6 | 26640 |
| 7 | 433440 |
| 8 | 8064000 |
| 9 | 168739200 |
| 10 | 3922732800 |
| 11 | 100310918400 |
| 12 | 2798327347200 |
| 13 | 84569169484800 |
| 14 | 2752393009766400 |
| 15 | 95978067913728000 |
| 16 | 3569951024640000000 |
| 17 | 141085196791418880000 |
| 18 | 5903699220162551808000 |
| 19 | 260764276201191899136000 |
| 20 | 12124067910801279713280000 |