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 | 1 |
| 1 | 1 |
| 2 | 4 |
| 3 | 15 |
| 4 | 84 |
| 5 | 555 |
| 6 | 4440 |
| 7 | 41265 |
| 8 | 438480 |
| 9 | 5240025 |
| 10 | 69582240 |
| 11 | 1016350335 |
| 12 | 16194911040 |
| 13 | 279560396115 |
| 14 | 5197054262400 |
| 15 | 103514720133825 |
| 16 | 2199255573715200 |
| 17 | 49645309340451825 |
| 18 | 1186599954328588800 |
| 19 | 29937224154635772975 |
| 20 | 795051251297099596800 |