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 | 1 |
| 2 | 1 |
| 3 | 3 |
| 4 | 18 |
| 5 | 120 |
| 6 | 1170 |
| 7 | 12600 |
| 8 | 176400 |
| 9 | 2608200 |
| 10 | 46607400 |
| 11 | 883159200 |
| 12 | 19429502400 |
| 13 | 447567120000 |
| 14 | 11629447830000 |
| 15 | 316028116404000 |
| 16 | 9516436753824000 |
| 17 | 297478346845680000 |
| 18 | 10151626256147376000 |
| 19 | 359237701318479984000 |
| 20 | 13733349319337487840000 |
| 21 | 542212802070902202240000 |