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 | 0 |
| 3 | 3 |
| 4 | 16 |
| 5 | 165 |
| 6 | 2016 |
| 7 | 30415 |
| 8 | 539904 |
| 9 | 11049129 |
| 10 | 256038400 |
| 11 | 6627314331 |
| 12 | 189517916160 |
| 13 | 5933803272397 |
| 14 | 201893195083776 |
| 15 | 7417376809230375 |
| 16 | 292648536838045696 |
| 17 | 12341039738944113105 |
| 18 | 553942486234823786496 |
| 19 | 26369048375194607316019 |
| 20 | 1326864458454400696320000 |