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 | 1 |
| 2 | 2 |
| 3 | 7 |
| 4 | 24 |
| 5 | 121 |
| 6 | 840 |
| 7 | 5041 |
| 8 | 40320 |
| 9 | 423361 |
| 10 | 3659040 |
| 11 | 39916801 |
| 12 | 558835200 |
| 13 | 6227020801 |
| 14 | 87195588480 |
| 15 | 1536517382401 |
| 16 | 20922789888000 |
| 17 | 355687428096001 |
| 18 | 7469453633241600 |
| 19 | 121645100408832001 |
| 20 | 2453176191578112000 |
| 21 | 59616236292028416001 |