A307454 a(n) is the number of canonical polygons with 2n edges having 2-fold rotational symmetry.
1, 3, 9, 33, 123, 513, 2077, 8759, 37184, 159508, 688446, 2993451, 13082724
Offset: 2
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.