In my Lecture Note \#8, Fig. 7.31 of the textbook is included. It seems that the mainlobe width m is equal to the frequency difference between two points A and B, where A is the frequency at which the passband ripple reaches maximum, and B is the frequency at which the stopband response is most negative. Is this a coincidence for this special case? Is it always true? Or, is it approximately true under certain conditions? Fi the ideal frequency response