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 | 1 |
| 2 | 2 |
| 3 | 6 |
| 4 | 28 |
| 5 | 160 |
| 6 | 1080 |
| 7 | 8540 |
| 8 | 78400 |
| 9 | 816480 |
| 10 | 9492000 |
| 11 | 122337600 |
| 12 | 1736380800 |
| 13 | 26930904000 |
| 14 | 453515462400 |
| 15 | 8254694448000 |
| 16 | 161734564992000 |
| 17 | 3397235761920000 |
| 18 | 76228261933824000 |
| 19 | 1821644243362944000 |
| 20 | 46233794313907200000 |
| 21 | 1242946827521118720000 |