Central frequency resonance. Coupling matrix means that when resonators resonate at the central frequency, energy can be reasonably stored and transferred between resonators through certain coupling structure design, so as to realize the filtering function at a specific frequency or specific requirements. Geometry is the study of spatial structure and properties. It is one of the most basic research contents in mathematics, which has the same important position as analysis and algebra and has a very close relationship. Geometry has a long history and rich contents.