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 | 22 |
| 5 | 100 |
| 6 | 548 |
| 7 | 3528 |
| 8 | 26136 |
| 9 | 219168 |
| 10 | 2053152 |
| 11 | 21257280 |
| 12 | 241087680 |
| 13 | 2972885760 |
| 14 | 39605518080 |
| 15 | 566931294720 |
| 16 | 8678326003200 |
| 17 | 141468564787200 |
| 18 | 2446811181158400 |
| 19 | 44753976117043200 |
| 20 | 863130293635276800 |