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 | 0 |
| 1 | 0 |
| 2 | 4 |
| 3 | 20 |
| 4 | 156 |
| 5 | 1024 |
| 6 | 7140 |
| 7 | 48620 |
| 8 | 334084 |
| 9 | 2287656 |
| 10 | 15685560 |
| 11 | 107495424 |
| 12 | 736823880 |
| 13 | 5050163160 |
| 14 | 34614602500 |
| 15 | 237251310140 |
| 16 | 1626146516820 |
| 17 | 11145769206784 |
| 18 | 76394251284780 |
| 19 | 523613954825156 |
| 20 | 3588903524021764 |