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## Existence of the Z Transform

The z transform of a finite-amplitude signal will always exist provided (1) the signal starts at a finite time and (2) it is asymptotically exponentially bounded, i.e., there exists a finite integer , and finite real numbers and , such that for all . The bounding exponential may even be growing with ( ). These are not the most general conditions for existence of the z transform, but they suffice for most practical purposes.

For a signal growing as , for , one would naturally expect the z transform to be defined only in the region of the complex plane. This is expected because the infinite series

requires to ensure convergence. Since for a decaying exponential, we see that the region of convergence of the transform of a decaying exponential always includes the unit circle of the plane.

More generally, it turns out that, in all cases of practical interest, the domain of can be extended to include the entire complex plane, except at isolated singular'' points7.2 at which approaches infinity (such as at when ). The mathematical technique for doing this is called analytic continuation, and it is described in §D.1 as applied to the Laplace transform (the continuous-time counterpart of the z transform). A point to note, however, is that in the extension region (all points such that in the above example), the signal component corresponding to each singularity inside the extension region is flipped'' in the time domain. That is, causal'' exponentials become anticausal'' exponentials, as discussed in §8.7.

The z transform is discussed more fully elsewhere [52,60], and we will derive below only what we will need.

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