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 | 4 |
| 3 | 15 |
| 4 | 80 |
| 5 | 490 |
| 6 | 3534 |
| 7 | 28938 |
| 8 | 266048 |
| 9 | 2710440 |
| 10 | 30311640 |
| 11 | 369127440 |
| 12 | 4862219592 |
| 13 | 68881435896 |
| 14 | 1044331262688 |
| 15 | 16872336545400 |
| 16 | 289380447338880 |
| 17 | 5251237965683520 |
| 18 | 100519388543098944 |
| 19 | 2024241909160239936 |
| 20 | 42780009017657888640 |
| 21 | 946724781741392908800 |