Theorem. re for whenever re for re, where is any positive real number.
Proof. We shall show that the change of variable , provides a conformal map from the z-plane to the s-plane that takes the region to the region re. The general formula for a bilinear conformal mapping of functions of a complex variable is given by
In general, a bilinear transformation maps circles and lines into circles and lines . We see that the choice of three specific points and their images determines the mapping for all and . We must have that the imaginary axis in the s-plane maps to the unit circle in the z-plane. That is, we may determine the mapping by three points of the form and . If we predispose one such mapping by choosing the pairs and , then we are left with transformations of the form
There is a bonus associated with the restriction that be real which is that
We have therefore proven
Theorem. PR PR, where is any positive real number.
The class of mappings of the form Eq.(6) which take the exterior of the unit circle to the right-half plane is larger than the class Eq.(7). For example, we may precede the transformation Eq.(7) by any conformal map which takes the unit disk to the unit disk, and these mappings have the algebraic form of a first order complex allpass whose zero lies inside the unit circle.
The bilinear transform is one which is used to map analog filters into digital filters. Another such mapping is called the matched transform . It also preserves the positive real property.
Theorem. is PR if is positive real in the analog sense, where is interpreted as the sampling period.
Proof. The mapping takes the right-half -plane to the outer disk in the -plane. Also is real if is real. Hence PR implies PR. (Note, however, that rational functions do not in general map to rational functions.)
These transformations allow application of the large battery of tests which exist for functions positive real in the right-half plane .