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 | 4 |
| 3 | 20 |
| 4 | 90 |
| 5 | 420 |
| 6 | 1910 |
| 7 | 8820 |
| 8 | 40590 |
| 9 | 187880 |
| 10 | 871494 |
| 11 | 4057620 |
| 12 | 18945960 |
| 13 | 88738020 |
| 14 | 416787030 |
| 15 | 1962922276 |
| 16 | 9268287390 |
| 17 | 43868210820 |
| 18 | 208109782580 |
| 19 | 989400443220 |
| 20 | 4713395564772 |
| 21 | 22497100553820 |
| 22 | 107572434560790 |
| 23 | 515241748300020 |