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 | 2 |
| 2 | 1 |
| 3 | 9 |
| 4 | 40 |
| 5 | 132 |
| 6 | 1032 |
| 7 | 7824 |
| 8 | 51912 |
| 9 | 530712 |
| 10 | 5619456 |
| 11 | 53529984 |
| 12 | 707802624 |
| 13 | 9591229440 |
| 14 | 119716185600 |
| 15 | 1938416832000 |
| 16 | 32061464064000 |
| 17 | 496487584512000 |
| 18 | 9509523337728000 |
| 19 | 185772505605120000 |
| 20 | 3434389946233344000 |