A143873 Number of ways of placing kings with no more than 1 mutual attack on an n X n chessboard symmetric about both diagonal and antidiagonal.
1, 2, 3, 5, 8, 21, 53, 158, 503, 2181, 9658, 53517, 313683, 2389270
Offset: 0
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.