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 | 0 |
| 2 | 2 |
| 3 | 6 |
| 4 | 66 |
| 5 | 450 |
| 6 | 4550 |
| 7 | 45570 |
| 8 | 543130 |
| 9 | 7044450 |
| 10 | 102177222 |
| 11 | 1621316466 |
| 12 | 28089336198 |
| 13 | 526810157874 |
| 14 | 10641259374174 |
| 15 | 230281144233426 |
| 16 | 5315651069181882 |
| 17 | 130370668142722722 |
| 18 | 3385534486308684710 |
| 19 | 92801581965119911026 |
| 20 | 2677687786557636155446 |
| 21 | 81124824677426691365490 |