### Convolution Theorem

The *convolution theorem for z transforms* states that for any (real or)
complex causal signals and ,

*convolution in the time domain is multiplication in the domain*,

*i.e.*,

*Proof: *

The convolution theorem provides a major cornerstone of linear systems theory. It implies, for example, that any stable causal LTI filter (recursive or nonrecursive) can be implemented by convolving the input signal with the impulse response of the filter, as shown in the next section.

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