Can anyone help me in finding the maximum magnitude error in the pass band and stop band of a fan-type quadrantally symmetric two-dimensional filter from its frequency response.

Exhaustive search in a quadrant?

Otherwise, I suppose that the details of the mathematical expression could be used to obtain the result analytically.  But, to do that, one would delve into those details I should think.

http://web.csulb.edu/~hreddy/Magazine_Symmetry.pdf is interesting.....