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 | 0 |
| 1 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 2 |
| 5 | 6 |
| 6 | 20 |
| 7 | 60 |
| 8 | 176 |
| 9 | 510 |
| 10 | 1484 |
| 11 | 4314 |
| 12 | 12624 |
| 13 | 37126 |
| 14 | 109864 |
| 15 | 326958 |
| 16 | 978528 |
| 17 | 2943384 |
| 18 | 8895792 |
| 19 | 27001378 |
| 20 | 82281216 |
| 21 | 251636434 |
| 22 | 772101086 |
| 23 | 2376186784 |
| 24 | 7333094178 |
| 25 | 22688117658 |
| 26 | 70360646672 |
| 27 | 218678194238 |
| 28 | 681016789056 |