Module

for

Branches of Complex Functions

2.4  Branches of Functions

In Section 2.2 we defined the principal square root function and investigated some of its properties.  We left unanswered some questions concerning the choices of square roots. We now look at these questions because they are similar to situations involving other elementary functions.

In our definition of a function in Section 2.1, we specified that each value of the independent variable in the domain is mapped onto one and only one value in the range.  As a result, we often talk about a single-valued function, which emphasizes the "only one" part of the definition and allows us to distinguish such functions from multiple-valued functions, which we now introduce.

Let denote a function whose domain is the set D and whose range is the set R.  If  w  is a value in the range, then there is an associated inverse function that assigns to each value w the value (or values) of z in D for which the equation holds.  But unless f takes on the value  w  at most once in D, then the inverse function g is necessarily many valued, and se say that g is a multivalued function.  For example, the inverse of the function is the square root function .  For each value z other than , then, the two points z and -z are mapped onto the same point ; hence g is in general a two-valued function.

The study of limits, continuity, and derivatives loses all meaning if an arbitrary or ambiguous assignment of function values is made.  For this reason, in Section 2.3 we did not allow multivalued functions to be considered when we defined these concepts.  When working with inverse functions, you have to specify carefully one of the many possible inverse values when constructing an inverse function, as when you determine implicit functions in calculus.  If the values of a function f are determined by an equation that they satisfy rather than by an explicit formula, then we say that the function is defined implicitly or that f is an implicit function. In the theory of complex variables we present a similar concept.

Let be a multiple-valued function.  A branch of f is any single-valued function that is continuous in some domain (except, perhaps, on the boundary).  At each point z in the domain, assigns one of the values of .  Associated with the branch of a function is the branch cut.

We now investigate the branches of the square root function.

Example 2.20.  We consider some branches of the two-valued square root function  ,  (where ).  Define the principal square root function as

(2-28)            ,

where    and   so that  .  The function is a branch of .  Using the same notation, we can find other branches of the square root function.  For example, if we let

(2-29)            ,

then

so can be thought of as "plus" and "minus" square root functions.  The negative real axis is called a branch cut for the functions .  Each point on the branch cut is a point of discontinuity for both functions .

Explore Solution 2.20.

Example 2.21.  Show that the function    is discontinuous along the negative real axis.

Solution.  Let    denote a negative real number.  We compute the limit as z approaches through the upper half-plane and the limit as z approaches through the lower half-plane .  In polar coordinates these limits are given by

,  and

.

As the two limits are distinct, the function is discontinuous at .

Remark 2.4  Likewise,   is discontinuous at .  The mappings  ,  ,  and the branch cut are illustrated in Figure 2.18.

Explore Solution 2.21.

(a)  The branch      (where   ).

(b)  The branch     (where   ).

Figure 2.18  The branches    and    of  .

We can construct other branches of the square root function by specifying that an argument of z given by    is to lie in the interval  .  The corresponding branch, denoted , is

(2-30)            ,

where    and  .

The branch cut for    is the ray  ,  which includes the origin.  The point , common to all branch cuts for the multivalued square root function, is called a branch point.  The mapping and its branch cut are illustrated in Figure 2.19.

Figure 2.19  The branch    of  .

The Riemann Surface for

A Riemann surface is a construct useful for visualizing a multivalued function.  It was introduced by Georg Friedrich Bernhard Riemann (1826-1866) in 1851.  The idea is ingenious - a geometric construction that permits surfaces to be the domain or range of a multivalued function.  Riemann surfaces depend on the function being investigated.  We now give a nontechnical formulation of the Riemann surface for the multivalued square root function.

Figure 2.A  A graphical view of the Riemann surface for  .

Consider , which has two values for any .  Each function in Figure 2.18 is single-valued on the domain formed by cutting the z plane along the negative x axis.  Let and be the domains of , respectively.  The range set for is the set consisting of the right half-plane, and the positive v axis; the range set for is the set consisting of the left half-plane and the negative v axis.  The sets are "glued together" along the positive v axis and the negative v axis to form the w plane with the origin deleted.

We stack directly above .  The edge of in the upper half-plane is joined to the edge of in the lower half-plane, and the edge of in the lower half-plane is joined to the edge of in the upper half-plane.  When these domains are glued together in this manner, they form R, which is a Riemann surface domain for the mapping .  The portions of that lie in are shown in Figure 2.20.

(a)  A portion of    and its image under  .

(b)  A portion of    and its image under  .

(c)  A portion of R and its image under  .

Figure 2.20  Formation of the Riemann surface for

Exploration

The beauty of this structure is that it makes this "full square root function" continuous for all .  Normally, the principal square root function would be discontinuous along the negative real axis, as points near but above that axis would get mapped to points close to , and points near but below the axis would get mapped to points close to .

Exercises for Section 2.4.  Branches of Functions

Graphics for Complex Functions

The Next Module is

The Reciprocal Transformation w=1/z

(c) 2012 John H. Mathews, Russell W. Howell