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 | 0 |
| 4 | 12 |
| 5 | 30 |
| 6 | 72 |
| 7 | 252 |
| 8 | 702 |
| 9 | 2268 |
| 10 | 6480 |
| 11 | 19602 |
| 12 | 59292 |
| 13 | 176904 |
| 14 | 532170 |
| 15 | 1592136 |
| 16 | 4785156 |
| 17 | 14351094 |
| 18 | 43040160 |
| 19 | 129146724 |
| 20 | 387400806 |
| 21 | 1162320516 |