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 | 6 |
| 4 | 6 |
| 5 | 30 |
| 6 | 72 |
| 7 | 252 |
| 8 | 756 |
| 9 | 2106 |
| 10 | 6642 |
| 11 | 19602 |
| 12 | 59292 |
| 13 | 176904 |
| 14 | 530712 |
| 15 | 1596510 |
| 16 | 4780782 |
| 17 | 14351094 |
| 18 | 43040160 |
| 19 | 129146724 |
| 20 | 387440172 |
| 21 | 1162202418 |