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 | 156 |
| 5 | 1560 |
| 6 | 18480 |
| 7 | 254520 |
| 8 | 3973200 |
| 9 | 68947200 |
| 10 | 1312748640 |
| 11 | 27175024800 |
| 12 | 607314818880 |
| 13 | 14566195163520 |
| 14 | 373027570755840 |
| 15 | 10154293067318400 |
| 16 | 292659790712889600 |
| 17 | 8899747730037964800 |
| 18 | 284685195814757337600 |
| 19 | 9553060139009702515200 |
| 20 | 335468448755976164428800 |