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) |
|---|---|
| 1 | 2 |
| 2 | 4 |
| 3 | 20 |
| 4 | 1540 |
| 5 | 609896980 |
| 6 | 37811003218473324699257860 |
| 7 | 9009555207802177724984164589516456320805205201729086740415363658290866918420 |