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 | 2 |
| 2 | 0 |
| 3 | 12 |
| 4 | 40 |
| 5 | 270 |
| 6 | 1848 |
| 7 | 14840 |
| 8 | 133488 |
| 9 | 1334970 |
| 10 | 14684560 |
| 11 | 176214852 |
| 12 | 2290793128 |
| 13 | 32071103750 |
| 14 | 481066556280 |
| 15 | 7697064900464 |
| 16 | 130850103307616 |
| 17 | 2355301859537394 |
| 18 | 44750735331210144 |
| 19 | 895014706624203260 |
| 20 | 18795308839108268040 |
| 21 | 413496794460381897342 |