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 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 48 |
| 5 | 400 |
| 6 | 4140 |
| 7 | 49644 |
| 8 | 681296 |
| 9 | 10515600 |
| 10 | 180349380 |
| 11 | 3402380740 |
| 12 | 70023004920 |
| 13 | 1561206957336 |
| 14 | 37485640585484 |
| 15 | 964345394431020 |
| 16 | 26462386594676640 |
| 17 | 771532717446066208 |
| 18 | 23817889096309943892 |
| 19 | 776127882633846005268 |