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) |
|---|---|
| 1 | 0 |
| 2 | 1 |
| 3 | 3 |
| 4 | 3 |
| 5 | 140 |
| 6 | 420 |
| 7 | 840 |
| 8 | 840 |
| 9 | 10810800 |
| 10 | 75675600 |
| 11 | 454053600 |
| 12 | 2270268000 |
| 13 | 9081072000 |
| 14 | 27243216000 |
| 15 | 54486432000 |
| 16 | 54486432000 |
| 17 | 52401161274029568000 |
| 18 | 786017419110443520000 |
| 19 | 11004243867546209280000 |
| 20 | 143055170278100720640000 |