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 | 0 |
| 2 | 0 |
| 3 | 2 |
| 4 | 8 |
| 5 | 44 |
| 6 | 264 |
| 7 | 1854 |
| 8 | 14832 |
| 9 | 133496 |
| 10 | 1334960 |
| 11 | 14684570 |
| 12 | 176214840 |
| 13 | 2290792932 |
| 14 | 32071101048 |
| 15 | 481066515734 |
| 16 | 7697064251744 |
| 17 | 130850092279664 |
| 18 | 2355301661033952 |
| 19 | 44750731559645106 |
| 20 | 895014631192902120 |