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 | 2 |
| 2 | 4 |
| 3 | 2 |
| 4 | 0 |
| 5 | 6 |
| 6 | 20 |
| 7 | 18 |
| 8 | 0 |
| 9 | 56 |
| 10 | 204 |
| 11 | 186 |
| 12 | 0 |
| 13 | 630 |
| 14 | 2340 |
| 15 | 2182 |
| 16 | 0 |
| 17 | 7710 |
| 18 | 29120 |
| 19 | 27594 |
| 20 | 0 |
| 21 | 99858 |
| 22 | 381300 |
| 23 | 364722 |
| 24 | 0 |
| 25 | 1342176 |
| 26 | 5162220 |
| 27 | 4971008 |
| 28 | 0 |
| 29 | 18512790 |
| 30 | 71582716 |
| 31 | 69273666 |
| 32 | 0 |
| 33 | 260300986 |
| 34 | 1010580540 |
| 35 | 981706806 |
| 36 | 0 |
| 37 | 3714566310 |
| 38 | 14467258260 |
| 39 | 14096302710 |
| 40 | 0 |