VW  CVM 1.1 
Our candidates for eigenfunctions of the Laplacian are
To complete invariance under the entire group pmg, it suffices to achieve invariance under a horizontal mirror whose axis is a quarter of the way up the cell. The functions will have to satisfy:
Working out the trigonometry (just apply the angle addition identity), we find that we require m to be odd in the sine terms, with m even in the cosine terms.

