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 | 10 |
| 4 | 70 |
| 5 | 210 |
| 6 | 4620 |
| 7 | 60060 |
| 8 | 60060 |
| 9 | 1021020 |
| 10 | 19399380 |
| 11 | 19399380 |
| 12 | 446185740 |
| 13 | 2230928700 |
| 14 | 13385572200 |
| 15 | 388181593800 |
| 16 | 12033629407800 |
| 17 | 12033629407800 |
| 18 | 12033629407800 |
| 19 | 445244288088600 |
| 20 | 445244288088600 |
| 21 | 18255015811632600 |