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 | 0 |
| 1 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 124 |
| 5 | 920 |
| 6 | 7940 |
| 7 | 78040 |
| 8 | 859580 |
| 9 | 10477880 |
| 10 | 139931620 |
| 11 | 2030707640 |
| 12 | 31805257340 |
| 13 | 534514790680 |
| 14 | 9591325648580 |
| 15 | 182974870484120 |
| 16 | 3697147584561340 |
| 17 | 78861451031150840 |
| 18 | 1770536585183202980 |
| 19 | 41729280102868841080 |
| 20 | 1030007496863617367420 |
| 21 | 26568602827124392999640 |