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 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 6 |
| 5 | 24 |
| 6 | 160 |
| 7 | 1140 |
| 8 | 8988 |
| 9 | 80864 |
| 10 | 809856 |
| 11 | 8907480 |
| 12 | 106877320 |
| 13 | 1389428832 |
| 14 | 19452141696 |
| 15 | 291781655984 |
| 16 | 4668504894480 |
| 17 | 79364592318720 |
| 18 | 1428562679845888 |
| 19 | 27142690734936864 |
| 20 | 542853814536802656 |
| 21 | 11399930109077490560 |
| 22 | 250798462399300784640 |