Even mathematically, I am not sure whether this is the viewpoint of wavenumber Prof.Wang mentioned, sin(kx+ly) can never be written as the linear combination of sin(kx) and sin(ly).

• 我们知道两列相互垂直的平面波不可能合成一个单一的第三个平面波，但是如果只从波数上考虑则可以这样做。