NB: We usually deal with causal and stable filters as
the real implementations have to be this. However, these properties
are not equivalent: a stable filter may be uncausal and a
causal filter may be unstable in general.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.