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 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 18 |
| 4 | 156 |
| 5 | 1560 |
| 6 | 18600 |
| 7 | 262080 |
| 8 | 4284000 |
| 9 | 79833600 |
| 10 | 1671788160 |
| 11 | 38878963200 |
| 12 | 994367404800 |
| 13 | 27741377664000 |
| 14 | 838406080512000 |
| 15 | 27287153858764800 |
| 16 | 951529253875200000 |
| 17 | 35392677532692480000 |
| 18 | 1398727294865252352000 |
| 19 | 58529661706809925632000 |
| 20 | 2585234639710708457472000 |