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 | 4 |
| 2 | 20 |
| 3 | 100 |
| 4 | 496 |
| 5 | 2464 |
| 6 | 12240 |
| 7 | 60800 |
| 8 | 302016 |
| 9 | 1500224 |
| 10 | 7452160 |
| 11 | 37017600 |
| 12 | 183879936 |
| 13 | 913398784 |
| 14 | 4537185280 |
| 15 | 22537856000 |
| 16 | 111953760256 |
| 17 | 556115206144 |
| 18 | 2762427289600 |
| 19 | 13721985024000 |
| 20 | 68162110078976 |
| 21 | 338586089570304 |