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 | 30 |
| 4 | 114 |
| 5 | 390 |
| 6 | 1266 |
| 7 | 3990 |
| 8 | 12354 |
| 9 | 37830 |
| 10 | 115026 |
| 11 | 348150 |
| 12 | 1050594 |
| 13 | 3164070 |
| 14 | 9516786 |
| 15 | 28599510 |
| 16 | 85896834 |
| 17 | 257887110 |
| 18 | 774054546 |
| 19 | 2322950070 |
| 20 | 6970423074 |
| 21 | 20914414950 |
| 22 | 62749536306 |
| 23 | 188261191830 |
| 24 | 564808741314 |
| 25 | 1694476555590 |
| 26 | 5083530330066 |
| 27 | 15250792316790 |