Complex Variables II Essentials
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Complex Variables II Essentials - Alan D. Solomon
MAPPINGS
CHAPTER 7
ELEMENTARY MAPPINGS AND THE MOBIUS TRANSFORMATION
7.1 REVIEW OF BASIC CONCEPTS
1. A function w = f(z) defines a mapping of its domain in the z-plane, onto its range R in the w-plane.
2. The image of a set S of the z-plane under a mapping w =f(z) is the set T of all points of the form w =f(z) for every point z of S. (Figure 7.1)
Fig. 7.1f(z) maps S onto T
3. The level lines of a function u = u(x, y) are the curves in the z-plane along which u is constant.
Fig. 7.2Level lines of a functionu(x,y)
In describing the effect of a mapping we will represent the images of representative points A, B, ... by A’, B’, ... as in Figure 7.1.
7.2 EXPANSIONS AND CONTRACTIONS
4. A function of the form
(7.1)
where c , a positive real number, is an expansion if c > 1 and a contraction if c < 1. The number c is referred to as the contraction or expansion factor for the mapping ƒ.
EXAMPLE 7.1
Under the expansion ƒ(z) = 2z, the rectangle
R: – 1
of the z-plane is mapped onto the rectangle
S: – 2
of the w = u + iv plane (Figure 7.3).
Fig. 7.3 The Image of a Rectangle Under w = 2z
EXAMPLE 7.2
Under the contraction w = ƒ(z) = z/2, the circle lzl = 3 of the z-plane is mapped onto the circle lwl = 3/2 of the w-plane (Figure 7.4).
Contractions and expansions map circles onto circles. Any circle of the z-plane has a polar representation
z = z0 + R0eiθ, 0≤θ ≤ 2π
with center z0 and radius R0. Under the contraction or expansion w =ƒ(z) = cz, real c > 0, the image of this circle is the set of points
w = cz = cz0 + cR0eiθ, 0≤θ ≤ 2π.
This is a circle in the w-plane with center cZ0 and radius cR0.
7.3 ROTATIONS
5. The mapping
(7.2)
for a given angle ø, is a rotation. This maps a point z onto a point w obtained by rotating the ray from the origin to z through the angle ø but conserving the distance to the origin (the modulus) (Figure 7.5). The ø is positive (negative) if the rotation is counterclockwise (clockwise).
EXAMPLE 7.3
In its polar representation,
Thus the mapping
is a rotation through π / 4= 45°. Therefore the image of the real axis z = x is the line u = ν of the w = u + iv plane, all bodies are rotated by 45° and the imaginary axis z = iy is mapped into the line u = −v (Figure 7.6).
A rotation through an angle ø has the same effect as that of rotating the entire z- plane through ø and placing it over the w-plane to obtain its u, v coordinates.
EXAMPLE 7.4
is a rotation through the angle 30°, or π / 6, since
7.4 TRANSLATIONS
6. For a given complex number zo the mapping
(7.3)
is a translation through the value zo
A translation moves the entire z-plane such that its axes maintain their original directions but the origin is now placed at the point zo of the w-plane (Figure 7.7); z = 0 is mapped into the point w = zo, circles are mapped into circles, lines into lines, and all orientations are maintained.
Fig. 7.7 The mapping w = z − (2 + 3i)
EXAMPLE 7.5
Under the translation w = ƒ(z) = z − (2 + 3i) the origin z = 0 is mapped onto the point w = −(2 + 3i), the circle
is mapped onto the circle
C ′: w = 2i +3eiθ – (2+3i)
= – (2+i) + 3eiθ ,
and lines are mapped onto lines preserving their original directions (Figure 7.7).
7.5 COMPOUND MAPPINGS
7. The compound function
(7.4)
with w = F(Z), and Z= G(z), defines a compound mapping from the z-plane to the w-plane. The mapping may be considered as one of two stages:
followed by
(Figure 7.8).
Fig. 7.8The compound functionw = F(G(z))
THEOREM
Let C be any complex number. Then the mapping
(7.5)
may be considered as the compound mapping of a rotation and a contraction or expansion.
PROOF
Suppose that C has the polar representation
C = coeiθ , co ≥ 0, 0 ≤