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 | 66 |
| 5 | 450 |
| 6 | 3690 |
| 7 | 35280 |
| 8 | 385560 |
| 9 | 6717480 |
| 10 | 96117000 |
| 11 | 1512819000 |
| 12 | 25975395600 |
| 13 | 483169486800 |
| 14 | 9678799930800 |
| 15 | 207733600074000 |
| 16 | 5243385642495000 |
| 17 | 128458209887007000 |
| 18 | 3332234177825553000 |
| 19 | 91241046790816923000 |
| 20 | 2629791992312269785000 |
| 21 | 79586945507310941700000 |