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 | 0 |
| 2 | 0 |
| 3 | 0 |
| 4 | 6 |
| 5 | 48 |
| 6 | 260 |
| 7 | 1200 |
| 8 | 5100 |
| 9 | 20720 |
| 10 | 81828 |
| 11 | 318000 |
| 12 | 1222870 |
| 13 | 4675440 |
| 14 | 17813820 |
| 15 | 67769504 |
| 16 | 257695800 |
| 17 | 980240880 |
| 18 | 3731732200 |
| 19 | 14222737200 |
| 20 | 54278498154 |
| 21 | 207438936800 |
| 22 | 793940157900 |
| 23 | 3043140078000 |
| 24 | 11681056021300 |
| 25 | 44900438149248 |
| 26 | 172824327151140 |
| 27 | 666070256468960 |