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 | 4 |
| 2 | 16 |
| 3 | 68 |
| 4 | 316 |
| 5 | 1616 |
| 6 | 9080 |
| 7 | 55800 |
| 8 | 373080 |
| 9 | 2699520 |
| 10 | 21035040 |
| 11 | 175708320 |
| 12 | 1566916320 |
| 13 | 14862171840 |
| 14 | 149429426880 |
| 15 | 1587766126080 |
| 16 | 17779538050560 |
| 17 | 209295747832320 |
| 18 | 2583920845209600 |
| 19 | 33389139008678400 |
| 20 | 450642388471395840 |
| 21 | 6342869733912760320 |