对于n=7,a(7)=2块板是彼此的镜像:
+---+---+---+---+---+---+---+
7 | | | * | | | | |
+---+---+---+---+---+---+---+
6 | | | | | | * | |
+---+---+---+---+---+---+---+
5 | * | | | | | | |
+---+---+---+---+---+---+---+
4 | | | | * | | | |
+---+---+---+---+---+---+---+.
3 | | | | | | | * |
+---+---+---+---+---+---+---+
2 | | * | | | | | |
+---+---+---+---+---+---+---+
1 | | | | | * | | |
+---+---+---+---+---+---+---+
A、B、C、D、E、F、G
+---+---+---+---+---+---+---+
7 | | | | | * | | |
+---+---+---+---+---+---+---+
6 | | * | | | | | |
+---+---+---+---+---+---+---+
5 | | | | | | | * |
+---+---+---+---+---+---+---+
4 | | | | * | | | |
+---+---+---+---+---+---+---+.
3 | * | | | | | | |
+---+---+---+---+---+---+---+
2 | | | | | | * | |
+---+---+---+---+---+---+---+
1 | | | * | | | | |
+---+---+---+---+---+---+---+
A、B、C、D、E、F、G