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 | 20 |
| 3 | 140 |
| 4 | 1008 |
| 5 | 7392 |
| 6 | 54912 |
| 7 | 411840 |
| 8 | 3111680 |
| 9 | 23648768 |
| 10 | 180590592 |
| 11 | 1384527872 |
| 12 | 10650214400 |
| 13 | 82158796800 |
| 14 | 635361361920 |
| 15 | 4924050554880 |
| 16 | 38233804308480 |
| 17 | 297374033510400 |
| 18 | 2316387208396800 |
| 19 | 18067820225495040 |
| 20 | 141101072237199360 |
| 21 | 1103153837490831360 |