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 | 1 |
| 2 | 4 |
| 3 | 16 |
| 4 | 80 |
| 5 | 420 |
| 6 | 2592 |
| 7 | 17352 |
| 8 | 132240 |
| 9 | 1117200 |
| 10 | 10559040 |
| 11 | 110276352 |
| 12 | 1268640000 |
| 13 | 15923168640 |
| 14 | 216767367936 |
| 15 | 3178157607936 |
| 16 | 49918919122944 |
| 17 | 835744605027840 |
| 18 | 14852897362759680 |
| 19 | 279172076525153280 |
| 20 | 5531978038112409600 |
| 21 | 115241366146485749760 |