Zeros and Poles


7.4  Singularities, Zeros, and Poles

    Recall that the point [Graphics:Images/SingularityZeroPoleMod._gr_1.gif] is called a singular point, or  singularity of the complex function f(z) if  f  is not analytic at  [Graphics:Images/SingularityZeroPoleMod._gr_2.gif],  but every neighborhood  [Graphics:Images/SingularityZeroPoleMod._gr_3.gif]  contains at least one point at which f(z) is analytic.  For example, the function  [Graphics:Images/SingularityZeroPoleMod._gr_4.gif]  is not analytic at  [Graphics:Images/SingularityZeroPoleMod._gr_5.gif],  but is analytic for all other values of z.  Thus the point  [Graphics:Images/SingularityZeroPoleMod._gr_6.gif]  is a singular point of f(z).  As another example,  consider  [Graphics:Images/SingularityZeroPoleMod._gr_7.gif].  We saw in Section 5.2 that g(z) is  analytic for all z except at the origin and at all points on the negative real-axis.  Thus, the origin and each point on the negative real axis is a singularity of  [Graphics:Images/SingularityZeroPoleMod._gr_8.gif].

    The point [Graphics:Images/SingularityZeroPoleMod._gr_9.gif] is called a isolated singularity of the complex function f(z) if  f  is not analytic at  [Graphics:Images/SingularityZeroPoleMod._gr_10.gif],  but there exists a real number  [Graphics:Images/SingularityZeroPoleMod._gr_11.gif]  such that f(z) is analytic everywhere in the punctured disk  [Graphics:Images/SingularityZeroPoleMod._gr_12.gif].  The function  [Graphics:Images/SingularityZeroPoleMod._gr_13.gif]  has an isolated singularity at  [Graphics:Images/SingularityZeroPoleMod._gr_14.gif].  

    The function  [Graphics:Images/SingularityZeroPoleMod._gr_15.gif],  however, the singularity at  [Graphics:Images/SingularityZeroPoleMod._gr_16.gif]  (or at any point of the negative real axis) that is not isolated, because any neighborhood of  contains points on the negative real axis, and  [Graphics:Images/SingularityZeroPoleMod._gr_17.gif]  is not analytic at those points.  Functions with isolated singularities have a Laurent series because the punctured disk   [Graphics:Images/SingularityZeroPoleMod._gr_18.gif]  is the same as the annulus  [Graphics:Images/SingularityZeroPoleMod._gr_19.gif].  The logarithm function  [Graphics:Images/SingularityZeroPoleMod._gr_20.gif]  does not have a Laurent series at any point  [Graphics:Images/SingularityZeroPoleMod._gr_21.gif]  on the negative real-axis.  We now look at this special case of Laurent's theorem in order to classify three types of isolated singularities.


Definition 7.5 (Removable Singularity, Pole of order k, Essential Singularity). Let f(z) have an isolated singularity at  [Graphics:Images/SingularityZeroPoleMod._gr_22.gif]  with Laurent series expansion  

            [Graphics:Images/SingularityZeroPoleMod._gr_23.gif]    valid for    [Graphics:Images/SingularityZeroPoleMod._gr_24.gif].  

Then we distinguish the following types of singularities at  [Graphics:Images/SingularityZeroPoleMod._gr_25.gif].  

(i)      If  [Graphics:Images/SingularityZeroPoleMod._gr_26.gif],  then we say that f(z) has a removable singularity at  [Graphics:Images/SingularityZeroPoleMod._gr_27.gif].  

(ii)      If k is a positive integer such that  [Graphics:Images/SingularityZeroPoleMod._gr_28.gif]  but [Graphics:Images/SingularityZeroPoleMod._gr_29.gif],  then we say that f(z) has a pole of order k at [Graphics:Images/SingularityZeroPoleMod._gr_30.gif].  

(iii)      If [Graphics:Images/SingularityZeroPoleMod._gr_31.gif] for infinitely many negative integers n, then we say that f(z) has an essential singularity at  [Graphics:Images/SingularityZeroPoleMod._gr_32.gif].  

    Let's investigate some examples of these three cases.

(i).  If f(z) has a removable singularity at [Graphics:Images/SingularityZeroPoleMod._gr_33.gif], then it has a Laurent series  

        [Graphics:Images/SingularityZeroPoleMod._gr_34.gif]    valid for    [Graphics:Images/SingularityZeroPoleMod._gr_35.gif].  

    Theorem 4.17 (see Section 4.4) implies that the power series for f(z) defines an analytic function in the disk  [Graphics:Images/SingularityZeroPoleMod._gr_36.gif].  
    If we use this series to define  [Graphics:Images/SingularityZeroPoleMod._gr_37.gif],  then the function f(z) becomes analytic at [Graphics:Images/SingularityZeroPoleMod._gr_38.gif], removing the singularity.  


    For example, consider the function  [Graphics:Images/SingularityZeroPoleMod._gr_39.gif].  It is undefined at [Graphics:Images/SingularityZeroPoleMod._gr_40.gif] and has an isolated singularity at [Graphics:Images/SingularityZeroPoleMod._gr_41.gif], as the Laurent series for f(z) is  


valid for [Graphics:Images/SingularityZeroPoleMod._gr_43.gif].  

We can remove this singularity if we define  [Graphics:Images/SingularityZeroPoleMod._gr_44.gif],  for then f(z) will be analytic at [Graphics:Images/SingularityZeroPoleMod._gr_45.gif] in accordance with Theorem 4.17 (see Section 4.4) .  

Exploration 1.


    Another example is  [Graphics:Images/SingularityZeroPoleMod._gr_75.gif],  which has an isolated singularity at the point [Graphics:Images/SingularityZeroPoleMod._gr_76.gif], as the Laurent series for g(z) is


valid for [Graphics:Images/SingularityZeroPoleMod._gr_78.gif].  If we define  [Graphics:Images/SingularityZeroPoleMod._gr_79.gif],  then g(z) will be analytic for all z.

Exploration 2.


(ii).  If f(z) has a pole of order k at [Graphics:Images/SingularityZeroPoleMod._gr_109.gif], the Laurent series for f(z) is

        [Graphics:Images/SingularityZeroPoleMod._gr_110.gif]    valid for    [Graphics:Images/SingularityZeroPoleMod._gr_111.gif].  

where  [Graphics:Images/SingularityZeroPoleMod._gr_112.gif].  


Extra Example 1. The following example will help this concept.  Consider the function  [Graphics:Images/SingularityZeroPoleMod._gr_113.gif].  The leading term in the Laurent series expansion  S(z)  is  [Graphics:Images/SingularityZeroPoleMod._gr_114.gif]  and  S(z)  goes to [Graphics:Images/SingularityZeroPoleMod._gr_115.gif] in the same manner as  [Graphics:Images/SingularityZeroPoleMod._gr_116.gif].            


Explore Solution Extra Example 1.



    Another example is;  


has a pole of order  [Graphics:Images/SingularityZeroPoleMod._gr_144.gif]  at  [Graphics:Images/SingularityZeroPoleMod._gr_145.gif].

Exploration 3.


    If f(z) has a pole of order 1 at [Graphics:Images/SingularityZeroPoleMod._gr_174.gif], we say that f(z) has a simple pole at [Graphics:Images/SingularityZeroPoleMod._gr_175.gif].   

    For example,  


has a simple pole at  [Graphics:Images/SingularityZeroPoleMod._gr_177.gif].  

Exploration 4.


(iii).  If infinitely many negative powers of [Graphics:Images/SingularityZeroPoleMod._gr_206.gif] occur in the Laurent series, then f(z) has an essential singularity at [Graphics:Images/SingularityZeroPoleMod._gr_207.gif].  For example,  


has an essential singularity at the origin.  

Exploration 5.


Definition 7.6 (Zero of order k).  A function f(z) analytic in  [Graphics:Images/SingularityZeroPoleMod._gr_237.gif]  has a zero of order k at the point  [Graphics:Images/SingularityZeroPoleMod._gr_238.gif]  if and only if  

            [Graphics:Images/SingularityZeroPoleMod._gr_239.gif],  and  [Graphics:Images/SingularityZeroPoleMod._gr_240.gif].

A zero of order one is sometimes called a simple zero.  


Theorem 7.10.  A function [Graphics:Images/SingularityZeroPoleMod._gr_241.gif] analytic in [Graphics:Images/SingularityZeroPoleMod._gr_242.gif] has a zero of order k at the point [Graphics:Images/SingularityZeroPoleMod._gr_243.gif]  iff its Taylor series given by [Graphics:Images/SingularityZeroPoleMod._gr_244.gif] has



Proof of Theorem 7.10 is in the book.
Complex Analysis for Mathematics and Engineering


Example 7.10.  From Theorem 7.10 we see that the function


has a zero of order [Graphics:Images/SingularityZeroPoleMod._gr_247.gif] at [Graphics:Images/SingularityZeroPoleMod._gr_248.gif].  Definition 7.6 confirms this fact because   


Then,  [Graphics:Images/SingularityZeroPoleMod._gr_250.gif],  but  [Graphics:Images/SingularityZeroPoleMod._gr_251.gif].  

Explore Solution 7.10.


Theorem 7.11.  Suppose f(z) is analytic in [Graphics:Images/SingularityZeroPoleMod._gr_278.gif].  Then f(z) has a zero of order k at the point [Graphics:Images/SingularityZeroPoleMod._gr_279.gif] if and only if it can be expressed in the form

(7-35)            [Graphics:Images/SingularityZeroPoleMod._gr_280.gif],

where g(z) is analytic at [Graphics:Images/SingularityZeroPoleMod._gr_281.gif].  


Proof of Theorem 7.11 is in the book.
Complex Analysis for Mathematics and Engineering


    An immediate consequence of Theorem 7.11 is Corollary 7.4.  The proof is left as an exercise.


Corollary 7.4.  If f(z) and g(z) are analytic at  [Graphics:Images/SingularityZeroPoleMod._gr_282.gif],  and have zeros of orders  m  and  n,  respectively at  [Graphics:Images/SingularityZeroPoleMod._gr_283.gif],  then their product  [Graphics:Images/SingularityZeroPoleMod._gr_284.gif]  has a zero of order  [Graphics:Images/SingularityZeroPoleMod._gr_285.gif].  



Example 7.11.  Let [Graphics:Images/SingularityZeroPoleMod._gr_286.gif].  Then f(z) can be factored as the product of  [Graphics:Images/SingularityZeroPoleMod._gr_287.gif]  and  [Graphics:Images/SingularityZeroPoleMod._gr_288.gif],  which have zeros of orders [Graphics:Images/SingularityZeroPoleMod._gr_289.gif] and [Graphics:Images/SingularityZeroPoleMod._gr_290.gif], respectively, at [Graphics:Images/SingularityZeroPoleMod._gr_291.gif].  
Hence  [Graphics:Images/SingularityZeroPoleMod._gr_292.gif] is a zero of order 4 of  f(z).

Explore Solution 7.11.


    Theorem 7.12 gives a useful way to characterize a pole.  


Theorem 7.12.  A function f(z) analytic in the punctured disk [Graphics:Images/SingularityZeroPoleMod._gr_324.gif] has a pole of order k at [Graphics:Images/SingularityZeroPoleMod._gr_325.gif] if and only if it can be expressed in the form
(7-37)            [Graphics:Images/SingularityZeroPoleMod._gr_326.gif],

where the function h(z) is analytic at the point [Graphics:Images/SingularityZeroPoleMod._gr_327.gif].  


Proof of Theorem 7.12 is in the book.
Complex Analysis for Mathematics and Engineering


    Corollaries 7.5-7.8 are useful in determining the order of a zero or a pole.  The proofs follow easily from Theorems 7.10 and 7.12 and are left as exercises.


Corollary 7.5.  If f(z) is analytic and has a zero of order k at the point  [Graphics:Images/SingularityZeroPoleMod._gr_328.gif],  then  [Graphics:Images/SingularityZeroPoleMod._gr_329.gif]  has a pole of order k at  [Graphics:Images/SingularityZeroPoleMod._gr_330.gif].  



Corollary 7.6. If  f(z) has a pole of order k at the point [Graphics:Images/SingularityZeroPoleMod._gr_331.gif], then [Graphics:Images/SingularityZeroPoleMod._gr_332.gif] has a removable singularity at [Graphics:Images/SingularityZeroPoleMod._gr_333.gif].  If we define [Graphics:Images/SingularityZeroPoleMod._gr_334.gif],  then g(z)  has a zero of order k at [Graphics:Images/SingularityZeroPoleMod._gr_335.gif].  



Corollary 7.7.  If f(z) and g(z) have poles of orders  m  and  n,  respectively at the point  [Graphics:Images/SingularityZeroPoleMod._gr_336.gif], then their product  [Graphics:Images/SingularityZeroPoleMod._gr_337.gif]  has a pole of order  [Graphics:Images/SingularityZeroPoleMod._gr_338.gif].  



Corollary 7.8.  Let f(z) and g(z) be analytic with  zeros of orders  m  and  n,  respectively at  [Graphics:Images/SingularityZeroPoleMod._gr_339.gif].  Then their quotient [Graphics:Images/SingularityZeroPoleMod._gr_340.gif]  has the following behavior:

(i)  If  [Graphics:Images/SingularityZeroPoleMod._gr_341.gif],  then h(z) has a removable singularity at  [Graphics:Images/SingularityZeroPoleMod._gr_342.gif].   If we define  [Graphics:Images/SingularityZeroPoleMod._gr_343.gif],  then h(z) has a zero of order  [Graphics:Images/SingularityZeroPoleMod._gr_344.gif].

(ii)  If  [Graphics:Images/SingularityZeroPoleMod._gr_345.gif],  then h(z) has a pole of order  [Graphics:Images/SingularityZeroPoleMod._gr_346.gif].

(iii)  If  [Graphics:Images/SingularityZeroPoleMod._gr_347.gif],  then h(z) has a removable singularity  at  [Graphics:Images/SingularityZeroPoleMod._gr_348.gif],  and can be defined so that h(z) is analytic at  [Graphics:Images/SingularityZeroPoleMod._gr_349.gif],  by  [Graphics:Images/SingularityZeroPoleMod._gr_350.gif].  



Example 7.12.  Locate the zeros and poles of  [Graphics:Images/SingularityZeroPoleMod._gr_351.gif],  and determine their order.

Solution.  In Section 5.4 we saw that the zeros of  [Graphics:Images/SingularityZeroPoleMod._gr_352.gif]  occur at the points  [Graphics:Images/SingularityZeroPoleMod._gr_353.gif],  where n is an integer.  Because  [Graphics:Images/SingularityZeroPoleMod._gr_354.gif],  the zeros of f(z) are simple.  Similarly, the function  [Graphics:Images/SingularityZeroPoleMod._gr_355.gif]  has simple zeros at the points [Graphics:Images/SingularityZeroPoleMod._gr_356.gif] and  [Graphics:Images/SingularityZeroPoleMod._gr_357.gif],  where n is an integer.  From the information given, we find that  [Graphics:Images/SingularityZeroPoleMod._gr_358.gif]  behaves as follows:

         i.    h(z)  has simple zeros at  [Graphics:Images/SingularityZeroPoleMod._gr_359.gif],  where  [Graphics:Images/SingularityZeroPoleMod._gr_360.gif];

         ii.   h(z)  has simple poles at  [Graphics:Images/SingularityZeroPoleMod._gr_361.gif],  where n is an integer;  and

         iii.  h(z)  is analytic at [Graphics:Images/SingularityZeroPoleMod._gr_362.gif] if we define  [Graphics:Images/SingularityZeroPoleMod._gr_363.gif].

Explore Solution 7.12.


Example 7.13.  Locate the poles of [Graphics:Images/SingularityZeroPoleMod._gr_402.gif], and specify their order.


Solution.  The roots of the quadratic equation  [Graphics:Images/SingularityZeroPoleMod._gr_404.gif]  occur at the points [Graphics:Images/SingularityZeroPoleMod._gr_405.gif].  If we replace z with [Graphics:Images/SingularityZeroPoleMod._gr_406.gif] in this equation, the function  [Graphics:Images/SingularityZeroPoleMod._gr_407.gif]  has simple zeros at the points  [Graphics:Images/SingularityZeroPoleMod._gr_408.gif].  Corollary 7.5 implies that g(z)  has simple poles at  [Graphics:Images/SingularityZeroPoleMod._gr_409.gif].

Explore Solution 7.13.


Example 7.14.  Locate the zeros and poles of [Graphics:Images/SingularityZeroPoleMod._gr_445.gif], and determine their order.

Solution. The function  [Graphics:Images/SingularityZeroPoleMod._gr_446.gif]  has a zero of order  [Graphics:Images/SingularityZeroPoleMod._gr_447.gif]  at  [Graphics:Images/SingularityZeroPoleMod._gr_448.gif]  and simple zeros at the points  [Graphics:Images/SingularityZeroPoleMod._gr_449.gif].  Corollary 7.5 implies that g(z) has a pole of order 3 at the point  [Graphics:Images/SingularityZeroPoleMod._gr_450.gif]  and simple poles at the points  [Graphics:Images/SingularityZeroPoleMod._gr_451.gif].  

Explore Solution 7.14.


Exercises for Section 7.4.  Singularities, Zeros, and Poles


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Laurent Series  

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