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 | 6 |
| 3 | 30 |
| 4 | 186 |
| 5 | 1110 |
| 6 | 6666 |
| 7 | 39990 |
| 8 | 239946 |
| 9 | 1439670 |
| 10 | 8638026 |
| 11 | 51828150 |
| 12 | 310968906 |
| 13 | 1865813430 |
| 14 | 11194880586 |
| 15 | 67169283510 |
| 16 | 403015701066 |
| 17 | 2418094206390 |
| 18 | 14508565238346 |
| 19 | 87051391430070 |
| 20 | 522308348580426 |
| 21 | 3133850091482550 |
| 22 | 18803100548895306 |
| 23 | 112818603293371830 |