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 | 6 |
| 3 | 6 |
| 4 | 18 |
| 5 | 90 |
| 6 | 150 |
| 7 | 420 |
| 8 | 1890 |
| 9 | 3570 |
| 10 | 10206 |
| 11 | 42966 |
| 12 | 87318 |
| 13 | 252252 |
| 14 | 1019304 |
| 15 | 2172456 |
| 16 | 6319170 |
| 17 | 24810786 |
| 18 | 54712086 |
| 19 | 159906318 |
| 20 | 614406078 |
| 21 | 1390381278 |
| 22 | 4077926034 |
| 23 | 15403838346 |
| 24 | 35579546262 |
| 25 | 104633453340 |
| 26 | 389788932240 |
| 27 | 915500037120 |
| 28 | 2698033909680 |
| 29 | 9934966920960 |