The Reciprocal Transformation w=1/z
2.5 The Reciprocal Transformation
The mapping is
called the reciprocal transformation and
maps the z-plane one-to-one and onto the w-plane except for the
point z=0, which has no image, and the
point w=0, which has no preimage or inverse
image. Use the exponential notation
in the w-plane. If , we
The geometric description of the reciprocal transformation is now evident. It is an inversion (that is, the modulus of () is the reciprocal of the modulus of z) followed by a reflection through the x axis. The ray , is mapped one-to-one and onto the ray . Points that lie inside the unit circle are mapped onto points that lie outside the unit circle and vice versa. The situation is illustrated in Figure 2.21.
Figure 2.21 The reciprocal transformation .
We can extend the system of complex numbers by joining to it an "ideal" point denoted by and called the point at infinity. This new set is called the extended complex plane. You will see shortly that the point has the property, loosely speaking, that iff .
An -neighborhood of the point at infinity is the set . The usual way to visualize the point at infinity is by using what we call the stereographic projection, which is attributed to Riemann. Let be a sphere of diameter 1 that is centered at the point in three-dimensional space where coordinates are specified by the triple of real numbers . Here the complex number will be associated with the point .
called the north pole of . If
we let z be a complex number and
consider the line segment L in
three-dimensional space that joins z
to the north pole , then
L intersects in exactly
one point . The
called the stereographic projection of the complex z
plane onto the Riemann
A point of unit modulus will correspond with . If z has modulus greater than 1, then will lie in the upper hemisphere where for points we have . If z has modulus less than 1, then will lie in the lower hemisphere where for points we have . The complex number corresponds with the south pole, .
Now you can see that indeed iff iff . Hence corresponds with the "ideal" point at infinity. The situation is shown in Figure 2.22.
Figure 2.22 The complex plane and the Riemann sphere .
Let us reconsider the
mapping . Let
us assign the images and to
the points and ,
respectively. We now write the reciprocal transformation
Note that the transformation is a one-to-one mapping of the extended complex z plane onto the extended complex w plane. Further, f is a continuous mapping from the extended z plane onto the extended w plane. We leave the details to you.
Extra Example. Investigate the limits of as .
Explore Solution for Extra Example.
Example 2.22. Show
that the image of the right half plane under
the mapping is
the closed disk in
Solution. Proceeding as we did in Example 2.7, we get the inverse mapping of as . Then
which describes the disk . As the reciprocal transformation is one-to-one, preimages of the points in the disk will lie in the right half-plane . Figure 2.23 illustrates this result.
Figure 2.23 The image of under the mapping .
Explore Solution 2.22.
Remark. Alas, there is a fly in the ointment here. As our notation indicates, and are not equivalent. The former implies the latter, but not conversely. That is, makes sense when , whereas does not. Yet Figure 2.23 seems to indicate that f maps onto the entire disk , including the point . Actually, it does not, because has no preimage in the complex plane. The way out of this dilemma is to use the complex point at infinity. It is that quantity that gets mapped to the point , for as we have already indicated, the preimage of 0 under the "extended" mapping is indeed .
Example 2.23. For
the transformation , find
the image of the portion of the right half
lies inside the closed disk .
Solution. Using the result of Example 2.22, we need only find the image of the closed disk and intersect it with the closed disk . To begin, we note that
Because , we
have, as before,
which is an inequality that determines the set of points in the w plane that lie on and outside the circle . Note that we do not have to deal with the point at infinity this time, as the last inequality is not satisfied when . When we intersect this set with , we get the crescent-shaped region shown in Figure 2.24.
Figure 2.24 The image of the half disk under is a crescent-shaped region.
Explore Solution 2.23.
To study images of "generalized circles,"
we consider the equation
where A, B, C, and D are real numbers. This equation represents either a circle or a line, depending on whether , respectively. Transforming the equation to polar coordinates gives
Using the polar coordinate form of the reciprocal transformation, we can express the image of the curve in the preceding equation as
which represents either a circle or a line, depending on whether , respectively. Therefore, we have shown that the reciprocal transformation carries the class of lines and circles onto itself.
2.24. Consider the
(a) Find the images of the vertical lines x = a. (b) Find the images the horizontal lines y = b.
Solution. Taking into account the point at infinity, we
see that the image of the line x=0 is the line u=0; that
is, the y axis is mapped onto the v axis.
Similarly, the x axis is mapped onto the u axis. Again, the inverse mapping is , so if , the vertical line is mapped onto the set of (u,v) points satisfying . For (u,v)~=(0,0), this outcome is equivalent to
which is the equation of a circle in the w plane with center and radius . The point at infinity is mapped to (u,v)=(0,0).
Similarly, the horizontal line is
mapped onto the circle
which has center and radius . Figure 2.25 illustrates the images of several lines.
Figure 2.25 The images of horizontal and vertical lines under the reciprocal transformation.
Explore Solution 2.24.
You are now ready to study Section 10.2 Bilinear Transformations - Mobius Transformations.
Exercises for Section 2.5. The Reciprocal Transformation
Mobius - Bilinear Transformation
The Next Module is
Differentiable and Analytic Functions
Return to the Complex Analysis Modules
Return to the Complex Analysis Project
This material is coordinated with our book Complex Analysis for Mathematics and Engineering.
(c) 2012 John H. Mathews, Russell W. Howell