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 | 2 |
| 3 | 6 |
| 4 | 36 |
| 5 | 260 |
| 6 | 1860 |
| 7 | 18732 |
| 8 | 162176 |
| 9 | 1774656 |
| 10 | 20909160 |
| 11 | 320781560 |
| 12 | 4493532912 |
| 13 | 72263548656 |
| 14 | 1081552065776 |
| 15 | 16682494491600 |
| 16 | 315573151180800 |
| 17 | 7042434106860032 |
| 18 | 151228601744085216 |
| 19 | 3306518316205792416 |