hi i am totally new to this forum and i am in desperate need of serious help with this question

What is the range of (real) values of r for which the filter is stable?



(1-r)^2 + (r^2 - 1)z^-2
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1 - 2r cos (WcT) z^-1 + r^2 z^-2

p.s. r^2 means to the power of 2 and so forth