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) |
|---|---|
| 1 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 6 |
| 5 | 24 |
| 6 | 84 |
| 7 | 312 |
| 8 | 1140 |
| 9 | 4248 |
| 10 | 15852 |
| 11 | 59688 |
| 12 | 225624 |
| 13 | 857424 |
| 14 | 3270216 |
| 15 | 12518472 |
| 16 | 48062520 |
| 17 | 185034360 |
| 18 | 714038448 |
| 19 | 2761343064 |
| 20 | 10698992304 |
| 21 | 41525207448 |
| 22 | 161418403008 |
| 23 | 628356425856 |
| 24 | 2449152822168 |
| 25 | 9557303840184 |