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 | 96 |
| 4 | 684 |
| 5 | 4320 |
| 6 | 35640 |
| 7 | 293760 |
| 8 | 2881440 |
| 9 | 29695680 |
| 10 | 345643200 |
| 11 | 4296499200 |
| 12 | 58518028800 |
| 13 | 851751936000 |
| 14 | 13338801100800 |
| 15 | 222145265664000 |
| 16 | 3934764739584000 |
| 17 | 73675580977152000 |
| 18 | 1455693852146688000 |
| 19 | 30238671742304256000 |
| 20 | 659012259039805440000 |