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 | 2 |
| 3 | 6 |
| 4 | 36 |
| 5 | 240 |
| 6 | 1800 |
| 7 | 15120 |
| 8 | 141120 |
| 9 | 1693440 |
| 10 | 21772800 |
| 11 | 299376000 |
| 12 | 4390848000 |
| 13 | 68497228800 |
| 14 | 1133317785600 |
| 15 | 19833061248000 |
| 16 | 396661224960000 |
| 17 | 8299373322240000 |
| 18 | 181400588328960000 |
| 19 | 4135933413900288000 |
| 20 | 98228418580131840000 |
| 21 | 2426819753156198400000 |