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 | 1 |
| 2 | 2 |
| 3 | 4 |
| 4 | 18 |
| 5 | 60 |
| 6 | 252 |
| 7 | 1860 |
| 8 | 9552 |
| 9 | 59616 |
| 10 | 565920 |
| 11 | 4051872 |
| 12 | 33805440 |
| 13 | 465239808 |
| 14 | 4294865664 |
| 15 | 35413136640 |
| 16 | 768372168960 |
| 17 | 8757710173440 |
| 18 | 79772814777600 |
| 19 | 1986906367584000 |
| 20 | 22082635812268800 |
| 21 | 280886415019776000 |
| 22 | 7683780010315046400 |