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 | 0 |
| 3 | 0 |
| 4 | 1 |
| 5 | 4 |
| 6 | 12 |
| 7 | 36 |
| 8 | 116 |
| 9 | 392 |
| 10 | 1350 |
| 11 | 4696 |
| 12 | 16500 |
| 13 | 58572 |
| 14 | 209824 |
| 15 | 757440 |
| 16 | 2752185 |
| 17 | 10057636 |
| 18 | 36943044 |
| 19 | 136319052 |
| 20 | 505086728 |
| 21 | 1878395920 |
| 22 | 7009239644 |
| 23 | 26235435248 |
| 24 | 98475145476 |
| 25 | 370584275964 |
| 26 | 1397918543552 |
| 27 | 5284861554816 |