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 | 20 |
| 4 | 116 |
| 5 | 744 |
| 6 | 5160 |
| 7 | 39360 |
| 8 | 350784 |
| 9 | 3749760 |
| 10 | 42940800 |
| 11 | 442713600 |
| 12 | 4650877440 |
| 13 | 109244298240 |
| 14 | 2833294464000 |
| 15 | -3487131648000 |
| 16 | -2166903606067200 |
| 17 | 51809012320665600 |
| 18 | 6808619561103360000 |
| 19 | -131306587205713920000 |
| 20 | -26982365129174827008000 |
| 21 | 595860034297401409536000 |