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 | 0 |
| 1 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 34 |
| 5 | 220 |
| 6 | 1808 |
| 7 | 17388 |
| 8 | 194724 |
| 9 | 2478096 |
| 10 | 35418192 |
| 11 | 561533280 |
| 12 | 9785418432 |
| 13 | 185921027136 |
| 14 | 3825633439392 |
| 15 | 84756646285920 |
| 16 | 2011657535668128 |
| 17 | 50924796197369088 |
| 18 | 1369659967551038976 |
| 19 | 39003791158314816768 |
| 20 | 1172394903935534452992 |
| 21 | 37094744191300029964800 |