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 | 1 |
| 2 | 4 |
| 3 | 20 |
| 4 | 128 |
| 5 | 1024 |
| 6 | 9984 |
| 7 | 115968 |
| 8 | 1572864 |
| 9 | 24477696 |
| 10 | 430571520 |
| 11 | 8452177920 |
| 12 | 183175741440 |
| 13 | 4343275192320 |
| 14 | 111817607086080 |
| 15 | 3105593229312000 |
| 16 | 92539365359616000 |
| 17 | 2944365169213440000 |
| 18 | 99619235621240832000 |
| 19 | 3571109329517936640000 |
| 20 | 135199252993504444416000 |
| 21 | 5390266968989421797376000 |