Extensions of procedure and numerical example: Having found the above bicubic prototype and its sufficiency condition, an arbitrary number of pole-zero pairs may now be added in a certain way, namely closer to the origin than —zx and farther from the origin than —z3, without invalidating the proposed procedure and without need of additional inequality requirements as regards the critical frequencies. To the left and to the right of the pole-zero plot for the bicubic prototype, the pole-zero configurations due to additional critical frequencies on the negative part of the real axis should exhibit the usual interlaced character, with zeros closest to and farthest from the origin. This extension may best be clarified by actually synthesising the following